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    Subject[PATCH v3 tip/core/rcu 37/40] srcu: Introduce CLASSIC_SRCU Kconfig option
    Date
    The TREE_SRCU rewrite is large and a bit on the non-simple side, so
    this commit helps reduce risk by allowing the old v4.11 SRCU algorithm
    to be selected using a new CLASSIC_SRCU Kconfig option that depends
    on RCU_EXPERT. The default is to use the new TREE_SRCU and TINY_SRCU
    algorithms, in order to help get these the testing that they need.
    However, if your users do not require the update-side scalability that
    is to be provided by TREE_SRCU, select RCU_EXPERT and then CLASSIC_SRCU
    to revert back to the old classic SRCU algorithm.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    include/linux/srcu.h | 2 +
    include/linux/srcuclassic.h | 101 ++++++++
    init/Kconfig | 21 +-
    kernel/rcu/Makefile | 3 +-
    kernel/rcu/rcutorture.c | 2 +-
    kernel/rcu/srcu.c | 347 ++++++++++++++-----------
    kernel/rcu/srcutree.c | 613 ++++++++++++++++++++++++++++++++++++++++++++
    7 files changed, 934 insertions(+), 155 deletions(-)
    create mode 100644 include/linux/srcuclassic.h
    create mode 100644 kernel/rcu/srcutree.c

    diff --git a/include/linux/srcu.h b/include/linux/srcu.h
    index 907f09b14eda..167ad8831aaf 100644
    --- a/include/linux/srcu.h
    +++ b/include/linux/srcu.h
    @@ -60,6 +60,8 @@ int init_srcu_struct(struct srcu_struct *sp);
    #include <linux/srcutiny.h>
    #elif defined(CONFIG_TREE_SRCU)
    #include <linux/srcutree.h>
    +#elif defined(CONFIG_CLASSIC_SRCU)
    +#include <linux/srcuclassic.h>
    #else
    #error "Unknown SRCU implementation specified to kernel configuration"
    #endif
    diff --git a/include/linux/srcuclassic.h b/include/linux/srcuclassic.h
    new file mode 100644
    index 000000000000..41cf99930f34
    --- /dev/null
    +++ b/include/linux/srcuclassic.h
    @@ -0,0 +1,101 @@
    +/*
    + * Sleepable Read-Copy Update mechanism for mutual exclusion,
    + * classic v4.11 variant.
    + *
    + * This program is free software; you can redistribute it and/or modify
    + * it under the terms of the GNU General Public License as published by
    + * the Free Software Foundation; either version 2 of the License, or
    + * (at your option) any later version.
    + *
    + * This program is distributed in the hope that it will be useful,
    + * but WITHOUT ANY WARRANTY; without even the implied warranty of
    + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    + * GNU General Public License for more details.
    + *
    + * You should have received a copy of the GNU General Public License
    + * along with this program; if not, you can access it online at
    + * http://www.gnu.org/licenses/gpl-2.0.html.
    + *
    + * Copyright (C) IBM Corporation, 2017
    + *
    + * Author: Paul McKenney <paulmck@us.ibm.com>
    + */
    +
    +#ifndef _LINUX_SRCU_CLASSIC_H
    +#define _LINUX_SRCU_CLASSIC_H
    +
    +struct srcu_array {
    + unsigned long lock_count[2];
    + unsigned long unlock_count[2];
    +};
    +
    +struct rcu_batch {
    + struct rcu_head *head, **tail;
    +};
    +
    +#define RCU_BATCH_INIT(name) { NULL, &(name.head) }
    +
    +struct srcu_struct {
    + unsigned long completed;
    + struct srcu_array __percpu *per_cpu_ref;
    + spinlock_t queue_lock; /* protect ->batch_queue, ->running */
    + bool running;
    + /* callbacks just queued */
    + struct rcu_batch batch_queue;
    + /* callbacks try to do the first check_zero */
    + struct rcu_batch batch_check0;
    + /* callbacks done with the first check_zero and the flip */
    + struct rcu_batch batch_check1;
    + struct rcu_batch batch_done;
    + struct delayed_work work;
    +#ifdef CONFIG_DEBUG_LOCK_ALLOC
    + struct lockdep_map dep_map;
    +#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
    +};
    +
    +void process_srcu(struct work_struct *work);
    +
    +#define __SRCU_STRUCT_INIT(name) \
    + { \
    + .completed = -300, \
    + .per_cpu_ref = &name##_srcu_array, \
    + .queue_lock = __SPIN_LOCK_UNLOCKED(name.queue_lock), \
    + .running = false, \
    + .batch_queue = RCU_BATCH_INIT(name.batch_queue), \
    + .batch_check0 = RCU_BATCH_INIT(name.batch_check0), \
    + .batch_check1 = RCU_BATCH_INIT(name.batch_check1), \
    + .batch_done = RCU_BATCH_INIT(name.batch_done), \
    + .work = __DELAYED_WORK_INITIALIZER(name.work, process_srcu, 0),\
    + __SRCU_DEP_MAP_INIT(name) \
    + }
    +
    +/*
    + * Define and initialize a srcu struct at build time.
    + * Do -not- call init_srcu_struct() nor cleanup_srcu_struct() on it.
    + *
    + * Note that although DEFINE_STATIC_SRCU() hides the name from other
    + * files, the per-CPU variable rules nevertheless require that the
    + * chosen name be globally unique. These rules also prohibit use of
    + * DEFINE_STATIC_SRCU() within a function. If these rules are too
    + * restrictive, declare the srcu_struct manually. For example, in
    + * each file:
    + *
    + * static struct srcu_struct my_srcu;
    + *
    + * Then, before the first use of each my_srcu, manually initialize it:
    + *
    + * init_srcu_struct(&my_srcu);
    + *
    + * See include/linux/percpu-defs.h for the rules on per-CPU variables.
    + */
    +#define __DEFINE_SRCU(name, is_static) \
    + static DEFINE_PER_CPU(struct srcu_array, name##_srcu_array);\
    + is_static struct srcu_struct name = __SRCU_STRUCT_INIT(name)
    +#define DEFINE_SRCU(name) __DEFINE_SRCU(name, /* not static */)
    +#define DEFINE_STATIC_SRCU(name) __DEFINE_SRCU(name, static)
    +
    +void synchronize_srcu_expedited(struct srcu_struct *sp);
    +void srcu_barrier(struct srcu_struct *sp);
    +unsigned long srcu_batches_completed(struct srcu_struct *sp);
    +
    +#endif
    diff --git a/init/Kconfig b/init/Kconfig
    index d269f2ca17b8..558cc3638ab9 100644
    --- a/init/Kconfig
    +++ b/init/Kconfig
    @@ -526,15 +526,32 @@ config SRCU
    permits arbitrary sleeping or blocking within RCU read-side critical
    sections.

    +config CLASSIC_SRCU
    + bool "Use v4.11 classic SRCU implementation"
    + default n
    + depends on RCU_EXPERT && SRCU
    + help
    + This option selects the traditional well-tested classic SRCU
    + implementation from v4.11, as might be desired for enterprise
    + Linux distributions. Without this option, the shiny new
    + Tiny SRCU and Tree SRCU implementations are used instead.
    + At some point, it is hoped that Tiny SRCU and Tree SRCU
    + will accumulate enough test time and confidence to allow
    + Classic SRCU to be dropped entirely.
    +
    + Say Y if you need a rock-solid SRCU.
    +
    + Say N if you would like help test Tree SRCU.
    +
    config TINY_SRCU
    bool
    - default y if TINY_RCU
    + default y if TINY_RCU && !CLASSIC_SRCU
    help
    This option selects the single-CPU non-preemptible version of SRCU.

    config TREE_SRCU
    bool
    - default y if !TINY_RCU
    + default y if !TINY_RCU && !CLASSIC_SRCU
    help
    This option selects the full-fledged version of SRCU.

    diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
    index b853214a2b99..158e6593d58c 100644
    --- a/kernel/rcu/Makefile
    +++ b/kernel/rcu/Makefile
    @@ -3,7 +3,8 @@
    KCOV_INSTRUMENT := n

    obj-y += update.o sync.o
    -obj-$(CONFIG_TREE_SRCU) += srcu.o
    +obj-$(CONFIG_CLASSIC_SRCU) += srcu.o
    +obj-$(CONFIG_TREE_SRCU) += srcutree.o
    obj-$(CONFIG_TINY_SRCU) += srcutiny.o
    obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
    obj-$(CONFIG_RCU_PERF_TEST) += rcuperf.o
    diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
    index 9cbb8a7b909d..6f344b6748a8 100644
    --- a/kernel/rcu/rcutorture.c
    +++ b/kernel/rcu/rcutorture.c
    @@ -562,7 +562,7 @@ static void srcu_torture_stats(void)
    int __maybe_unused cpu;
    int idx;

    -#ifdef CONFIG_TREE_SRCU
    +#if defined(CONFIG_TREE_SRCU) || defined(CONFIG_CLASSIC_SRCU)
    idx = srcu_ctlp->completed & 0x1;
    pr_alert("%s%s Tree SRCU per-CPU(idx=%d):",
    torture_type, TORTURE_FLAG, idx);
    diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
    index 3cfcc59bddf3..584d8a983883 100644
    --- a/kernel/rcu/srcu.c
    +++ b/kernel/rcu/srcu.c
    @@ -36,16 +36,75 @@
    #include <linux/delay.h>
    #include <linux/srcu.h>

    -#include <linux/rcu_node_tree.h>
    #include "rcu.h"

    +/*
    + * Initialize an rcu_batch structure to empty.
    + */
    +static inline void rcu_batch_init(struct rcu_batch *b)
    +{
    + b->head = NULL;
    + b->tail = &b->head;
    +}
    +
    +/*
    + * Enqueue a callback onto the tail of the specified rcu_batch structure.
    + */
    +static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
    +{
    + *b->tail = head;
    + b->tail = &head->next;
    +}
    +
    +/*
    + * Is the specified rcu_batch structure empty?
    + */
    +static inline bool rcu_batch_empty(struct rcu_batch *b)
    +{
    + return b->tail == &b->head;
    +}
    +
    +/*
    + * Remove the callback at the head of the specified rcu_batch structure
    + * and return a pointer to it, or return NULL if the structure is empty.
    + */
    +static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
    +{
    + struct rcu_head *head;
    +
    + if (rcu_batch_empty(b))
    + return NULL;
    +
    + head = b->head;
    + b->head = head->next;
    + if (b->tail == &head->next)
    + rcu_batch_init(b);
    +
    + return head;
    +}
    +
    +/*
    + * Move all callbacks from the rcu_batch structure specified by "from" to
    + * the structure specified by "to".
    + */
    +static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
    +{
    + if (!rcu_batch_empty(from)) {
    + *to->tail = from->head;
    + to->tail = from->tail;
    + rcu_batch_init(from);
    + }
    +}
    +
    static int init_srcu_struct_fields(struct srcu_struct *sp)
    {
    sp->completed = 0;
    - sp->srcu_gp_seq = 0;
    - atomic_set(&sp->srcu_exp_cnt, 0);
    spin_lock_init(&sp->queue_lock);
    - rcu_segcblist_init(&sp->srcu_cblist);
    + sp->running = false;
    + rcu_batch_init(&sp->batch_queue);
    + rcu_batch_init(&sp->batch_check0);
    + rcu_batch_init(&sp->batch_check1);
    + rcu_batch_init(&sp->batch_done);
    INIT_DELAYED_WORK(&sp->work, process_srcu);
    sp->per_cpu_ref = alloc_percpu(struct srcu_array);
    return sp->per_cpu_ref ? 0 : -ENOMEM;
    @@ -180,8 +239,6 @@ static bool srcu_readers_active(struct srcu_struct *sp)
    return sum;
    }

    -#define SRCU_INTERVAL 1
    -
    /**
    * cleanup_srcu_struct - deconstruct a sleep-RCU structure
    * @sp: structure to clean up.
    @@ -197,16 +254,8 @@ static bool srcu_readers_active(struct srcu_struct *sp)
    */
    void cleanup_srcu_struct(struct srcu_struct *sp)
    {
    - WARN_ON_ONCE(atomic_read(&sp->srcu_exp_cnt));
    if (WARN_ON(srcu_readers_active(sp)))
    return; /* Leakage unless caller handles error. */
    - if (WARN_ON(!rcu_segcblist_empty(&sp->srcu_cblist)))
    - return; /* Leakage unless caller handles error. */
    - flush_delayed_work(&sp->work);
    - if (WARN_ON(rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) != SRCU_STATE_IDLE)) {
    - pr_info("cleanup_srcu_struct: Active srcu_struct %lu CBs %c state: %d\n", rcu_segcblist_n_cbs(&sp->srcu_cblist), ".E"[rcu_segcblist_empty(&sp->srcu_cblist)], rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)));
    - return; /* Caller forgot to stop doing call_srcu()? */
    - }
    free_percpu(sp->per_cpu_ref);
    sp->per_cpu_ref = NULL;
    }
    @@ -245,36 +294,26 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
    * We use an adaptive strategy for synchronize_srcu() and especially for
    * synchronize_srcu_expedited(). We spin for a fixed time period
    * (defined below) to allow SRCU readers to exit their read-side critical
    - * sections. If there are still some readers after a few microseconds,
    - * we repeatedly block for 1-millisecond time periods.
    + * sections. If there are still some readers after 10 microseconds,
    + * we repeatedly block for 1-millisecond time periods. This approach
    + * has done well in testing, so there is no need for a config parameter.
    */
    #define SRCU_RETRY_CHECK_DELAY 5
    +#define SYNCHRONIZE_SRCU_TRYCOUNT 2
    +#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12

    /*
    - * Start an SRCU grace period.
    - */
    -static void srcu_gp_start(struct srcu_struct *sp)
    -{
    - int state;
    -
    - rcu_segcblist_accelerate(&sp->srcu_cblist,
    - rcu_seq_snap(&sp->srcu_gp_seq));
    - rcu_seq_start(&sp->srcu_gp_seq);
    - state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
    - WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
    -}
    -
    -/*
    - * Wait until all readers counted by array index idx complete, but
    - * loop an additional time if there is an expedited grace period pending.
    - * The caller must ensure that ->completed is not changed while checking.
    + * @@@ Wait until all pre-existing readers complete. Such readers
    + * will have used the index specified by "idx".
    + * the caller should ensures the ->completed is not changed while checking
    + * and idx = (->completed & 1) ^ 1
    */
    static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
    {
    for (;;) {
    if (srcu_readers_active_idx_check(sp, idx))
    return true;
    - if (--trycount + !!atomic_read(&sp->srcu_exp_cnt) <= 0)
    + if (--trycount <= 0)
    return false;
    udelay(SRCU_RETRY_CHECK_DELAY);
    }
    @@ -300,19 +339,6 @@ static void srcu_flip(struct srcu_struct *sp)
    }

    /*
    - * End an SRCU grace period.
    - */
    -static void srcu_gp_end(struct srcu_struct *sp)
    -{
    - rcu_seq_end(&sp->srcu_gp_seq);
    -
    - spin_lock_irq(&sp->queue_lock);
    - rcu_segcblist_advance(&sp->srcu_cblist,
    - rcu_seq_current(&sp->srcu_gp_seq));
    - spin_unlock_irq(&sp->queue_lock);
    -}
    -
    -/*
    * Enqueue an SRCU callback on the specified srcu_struct structure,
    * initiating grace-period processing if it is not already running.
    *
    @@ -348,24 +374,26 @@ void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
    head->func = func;
    spin_lock_irqsave(&sp->queue_lock, flags);
    smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */
    - rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
    - if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_IDLE) {
    - srcu_gp_start(sp);
    + rcu_batch_queue(&sp->batch_queue, head);
    + if (!sp->running) {
    + sp->running = true;
    queue_delayed_work(system_power_efficient_wq, &sp->work, 0);
    }
    spin_unlock_irqrestore(&sp->queue_lock, flags);
    }
    EXPORT_SYMBOL_GPL(call_srcu);

    -static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
    +static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
    +static void srcu_reschedule(struct srcu_struct *sp);

    /*
    * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
    */
    -static void __synchronize_srcu(struct srcu_struct *sp)
    +static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
    {
    struct rcu_synchronize rcu;
    struct rcu_head *head = &rcu.head;
    + bool done = false;

    RCU_LOCKDEP_WARN(lock_is_held(&sp->dep_map) ||
    lock_is_held(&rcu_bh_lock_map) ||
    @@ -373,8 +401,6 @@ static void __synchronize_srcu(struct srcu_struct *sp)
    lock_is_held(&rcu_sched_lock_map),
    "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");

    - if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
    - return;
    might_sleep();
    init_completion(&rcu.completion);

    @@ -382,47 +408,31 @@ static void __synchronize_srcu(struct srcu_struct *sp)
    head->func = wakeme_after_rcu;
    spin_lock_irq(&sp->queue_lock);
    smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */
    - if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_IDLE) {
    + if (!sp->running) {
    /* steal the processing owner */
    - rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
    - srcu_gp_start(sp);
    + sp->running = true;
    + rcu_batch_queue(&sp->batch_check0, head);
    spin_unlock_irq(&sp->queue_lock);
    +
    + srcu_advance_batches(sp, trycount);
    + if (!rcu_batch_empty(&sp->batch_done)) {
    + BUG_ON(sp->batch_done.head != head);
    + rcu_batch_dequeue(&sp->batch_done);
    + done = true;
    + }
    /* give the processing owner to work_struct */
    - srcu_reschedule(sp, 0);
    + srcu_reschedule(sp);
    } else {
    - rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
    + rcu_batch_queue(&sp->batch_queue, head);
    spin_unlock_irq(&sp->queue_lock);
    }

    - wait_for_completion(&rcu.completion);
    - smp_mb(); /* Caller's later accesses after GP. */
    -}
    -
    -/**
    - * synchronize_srcu_expedited - Brute-force SRCU grace period
    - * @sp: srcu_struct with which to synchronize.
    - *
    - * Wait for an SRCU grace period to elapse, but be more aggressive about
    - * spinning rather than blocking when waiting.
    - *
    - * Note that synchronize_srcu_expedited() has the same deadlock and
    - * memory-ordering properties as does synchronize_srcu().
    - */
    -void synchronize_srcu_expedited(struct srcu_struct *sp)
    -{
    - bool do_norm = rcu_gp_is_normal();
    -
    - if (!do_norm) {
    - atomic_inc(&sp->srcu_exp_cnt);
    - smp_mb__after_atomic(); /* increment before GP. */
    - }
    - __synchronize_srcu(sp);
    - if (!do_norm) {
    - smp_mb__before_atomic(); /* GP before decrement. */
    - atomic_dec(&sp->srcu_exp_cnt);
    + if (!done) {
    + wait_for_completion(&rcu.completion);
    + smp_mb(); /* Caller's later accesses after GP. */
    }
    +
    }
    -EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);

    /**
    * synchronize_srcu - wait for prior SRCU read-side critical-section completion
    @@ -465,14 +475,29 @@ EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
    */
    void synchronize_srcu(struct srcu_struct *sp)
    {
    - if (rcu_gp_is_expedited())
    - synchronize_srcu_expedited(sp);
    - else
    - __synchronize_srcu(sp);
    + __synchronize_srcu(sp, (rcu_gp_is_expedited() && !rcu_gp_is_normal())
    + ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
    + : SYNCHRONIZE_SRCU_TRYCOUNT);
    }
    EXPORT_SYMBOL_GPL(synchronize_srcu);

    /**
    + * synchronize_srcu_expedited - Brute-force SRCU grace period
    + * @sp: srcu_struct with which to synchronize.
    + *
    + * Wait for an SRCU grace period to elapse, but be more aggressive about
    + * spinning rather than blocking when waiting.
    + *
    + * Note that synchronize_srcu_expedited() has the same deadlock and
    + * memory-ordering properties as does synchronize_srcu().
    + */
    +void synchronize_srcu_expedited(struct srcu_struct *sp)
    +{
    + __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
    +}
    +EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
    +
    +/**
    * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
    * @sp: srcu_struct on which to wait for in-flight callbacks.
    */
    @@ -495,13 +520,29 @@ unsigned long srcu_batches_completed(struct srcu_struct *sp)
    }
    EXPORT_SYMBOL_GPL(srcu_batches_completed);

    +#define SRCU_CALLBACK_BATCH 10
    +#define SRCU_INTERVAL 1
    +
    +/*
    + * Move any new SRCU callbacks to the first stage of the SRCU grace
    + * period pipeline.
    + */
    +static void srcu_collect_new(struct srcu_struct *sp)
    +{
    + if (!rcu_batch_empty(&sp->batch_queue)) {
    + spin_lock_irq(&sp->queue_lock);
    + rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
    + spin_unlock_irq(&sp->queue_lock);
    + }
    +}
    +
    /*
    * Core SRCU state machine. Advance callbacks from ->batch_check0 to
    * ->batch_check1 and then to ->batch_done as readers drain.
    */
    -static void srcu_advance_batches(struct srcu_struct *sp)
    +static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
    {
    - int idx;
    + int idx = 1 ^ (sp->completed & 1);

    /*
    * Because readers might be delayed for an extended period after
    @@ -509,44 +550,50 @@ static void srcu_advance_batches(struct srcu_struct *sp)
    * might well be readers using both idx=0 and idx=1. We therefore
    * need to wait for readers to clear from both index values before
    * invoking a callback.
    - *
    - * The load-acquire ensures that we see the accesses performed
    - * by the prior grace period.
    */
    - idx = rcu_seq_state(smp_load_acquire(&sp->srcu_gp_seq)); /* ^^^ */
    - if (idx == SRCU_STATE_IDLE) {
    - spin_lock_irq(&sp->queue_lock);
    - if (rcu_segcblist_empty(&sp->srcu_cblist)) {
    - spin_unlock_irq(&sp->queue_lock);
    - return;
    - }
    - idx = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
    - if (idx == SRCU_STATE_IDLE)
    - srcu_gp_start(sp);
    - spin_unlock_irq(&sp->queue_lock);
    - if (idx != SRCU_STATE_IDLE)
    - return; /* Someone else started the grace period. */
    - }

    - if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
    - idx = 1 ^ (sp->completed & 1);
    - if (!try_check_zero(sp, idx, 1))
    - return; /* readers present, retry later. */
    - srcu_flip(sp);
    - rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2);
    - }
    + if (rcu_batch_empty(&sp->batch_check0) &&
    + rcu_batch_empty(&sp->batch_check1))
    + return; /* no callbacks need to be advanced */

    - if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {
    + if (!try_check_zero(sp, idx, trycount))
    + return; /* failed to advance, will try after SRCU_INTERVAL */

    - /*
    - * SRCU read-side critical sections are normally short,
    - * so check at least twice in quick succession after a flip.
    - */
    - idx = 1 ^ (sp->completed & 1);
    - if (!try_check_zero(sp, idx, 2))
    - return; /* readers present, retry after later. */
    - srcu_gp_end(sp);
    - }
    + /*
    + * The callbacks in ->batch_check1 have already done with their
    + * first zero check and flip back when they were enqueued on
    + * ->batch_check0 in a previous invocation of srcu_advance_batches().
    + * (Presumably try_check_zero() returned false during that
    + * invocation, leaving the callbacks stranded on ->batch_check1.)
    + * They are therefore ready to invoke, so move them to ->batch_done.
    + */
    + rcu_batch_move(&sp->batch_done, &sp->batch_check1);
    +
    + if (rcu_batch_empty(&sp->batch_check0))
    + return; /* no callbacks need to be advanced */
    + srcu_flip(sp);
    +
    + /*
    + * The callbacks in ->batch_check0 just finished their
    + * first check zero and flip, so move them to ->batch_check1
    + * for future checking on the other idx.
    + */
    + rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
    +
    + /*
    + * SRCU read-side critical sections are normally short, so check
    + * at least twice in quick succession after a flip.
    + */
    + trycount = trycount < 2 ? 2 : trycount;
    + if (!try_check_zero(sp, idx^1, trycount))
    + return; /* failed to advance, will try after SRCU_INTERVAL */
    +
    + /*
    + * The callbacks in ->batch_check1 have now waited for all
    + * pre-existing readers using both idx values. They are therefore
    + * ready to invoke, so move them to ->batch_done.
    + */
    + rcu_batch_move(&sp->batch_done, &sp->batch_check1);
    }

    /*
    @@ -557,48 +604,45 @@ static void srcu_advance_batches(struct srcu_struct *sp)
    */
    static void srcu_invoke_callbacks(struct srcu_struct *sp)
    {
    - struct rcu_cblist ready_cbs;
    - struct rcu_head *rhp;
    + int i;
    + struct rcu_head *head;

    - spin_lock_irq(&sp->queue_lock);
    - if (!rcu_segcblist_ready_cbs(&sp->srcu_cblist)) {
    - spin_unlock_irq(&sp->queue_lock);
    - return;
    - }
    - rcu_cblist_init(&ready_cbs);
    - rcu_segcblist_extract_done_cbs(&sp->srcu_cblist, &ready_cbs);
    - spin_unlock_irq(&sp->queue_lock);
    - rhp = rcu_cblist_dequeue(&ready_cbs);
    - for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
    + for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
    + head = rcu_batch_dequeue(&sp->batch_done);
    + if (!head)
    + break;
    local_bh_disable();
    - rhp->func(rhp);
    + head->func(head);
    local_bh_enable();
    }
    - spin_lock_irq(&sp->queue_lock);
    - rcu_segcblist_insert_count(&sp->srcu_cblist, &ready_cbs);
    - spin_unlock_irq(&sp->queue_lock);
    }

    /*
    * Finished one round of SRCU grace period. Start another if there are
    * more SRCU callbacks queued, otherwise put SRCU into not-running state.
    */
    -static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay)
    +static void srcu_reschedule(struct srcu_struct *sp)
    {
    bool pending = true;
    - int state;

    - if (rcu_segcblist_empty(&sp->srcu_cblist)) {
    + if (rcu_batch_empty(&sp->batch_done) &&
    + rcu_batch_empty(&sp->batch_check1) &&
    + rcu_batch_empty(&sp->batch_check0) &&
    + rcu_batch_empty(&sp->batch_queue)) {
    spin_lock_irq(&sp->queue_lock);
    - state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
    - if (rcu_segcblist_empty(&sp->srcu_cblist) &&
    - state == SRCU_STATE_IDLE)
    + if (rcu_batch_empty(&sp->batch_done) &&
    + rcu_batch_empty(&sp->batch_check1) &&
    + rcu_batch_empty(&sp->batch_check0) &&
    + rcu_batch_empty(&sp->batch_queue)) {
    + sp->running = false;
    pending = false;
    + }
    spin_unlock_irq(&sp->queue_lock);
    }

    if (pending)
    - queue_delayed_work(system_power_efficient_wq, &sp->work, delay);
    + queue_delayed_work(system_power_efficient_wq,
    + &sp->work, SRCU_INTERVAL);
    }

    /*
    @@ -610,8 +654,9 @@ void process_srcu(struct work_struct *work)

    sp = container_of(work, struct srcu_struct, work.work);

    - srcu_advance_batches(sp);
    + srcu_collect_new(sp);
    + srcu_advance_batches(sp, 1);
    srcu_invoke_callbacks(sp);
    - srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
    + srcu_reschedule(sp);
    }
    EXPORT_SYMBOL_GPL(process_srcu);
    diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
    new file mode 100644
    index 000000000000..da676b0d016b
    --- /dev/null
    +++ b/kernel/rcu/srcutree.c
    @@ -0,0 +1,613 @@
    +/*
    + * Sleepable Read-Copy Update mechanism for mutual exclusion.
    + *
    + * This program is free software; you can redistribute it and/or modify
    + * it under the terms of the GNU General Public License as published by
    + * the Free Software Foundation; either version 2 of the License, or
    + * (at your option) any later version.
    + *
    + * This program is distributed in the hope that it will be useful,
    + * but WITHOUT ANY WARRANTY; without even the implied warranty of
    + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    + * GNU General Public License for more details.
    + *
    + * You should have received a copy of the GNU General Public License
    + * along with this program; if not, you can access it online at
    + * http://www.gnu.org/licenses/gpl-2.0.html.
    + *
    + * Copyright (C) IBM Corporation, 2006
    + * Copyright (C) Fujitsu, 2012
    + *
    + * Author: Paul McKenney <paulmck@us.ibm.com>
    + * Lai Jiangshan <laijs@cn.fujitsu.com>
    + *
    + * For detailed explanation of Read-Copy Update mechanism see -
    + * Documentation/RCU/ *.txt
    + *
    + */
    +
    +#include <linux/export.h>
    +#include <linux/mutex.h>
    +#include <linux/percpu.h>
    +#include <linux/preempt.h>
    +#include <linux/rcupdate_wait.h>
    +#include <linux/sched.h>
    +#include <linux/smp.h>
    +#include <linux/delay.h>
    +#include <linux/srcu.h>
    +
    +#include <linux/rcu_node_tree.h>
    +#include "rcu.h"
    +
    +static int init_srcu_struct_fields(struct srcu_struct *sp)
    +{
    + sp->completed = 0;
    + sp->srcu_gp_seq = 0;
    + atomic_set(&sp->srcu_exp_cnt, 0);
    + spin_lock_init(&sp->queue_lock);
    + rcu_segcblist_init(&sp->srcu_cblist);
    + INIT_DELAYED_WORK(&sp->work, process_srcu);
    + sp->per_cpu_ref = alloc_percpu(struct srcu_array);
    + return sp->per_cpu_ref ? 0 : -ENOMEM;
    +}
    +
    +#ifdef CONFIG_DEBUG_LOCK_ALLOC
    +
    +int __init_srcu_struct(struct srcu_struct *sp, const char *name,
    + struct lock_class_key *key)
    +{
    + /* Don't re-initialize a lock while it is held. */
    + debug_check_no_locks_freed((void *)sp, sizeof(*sp));
    + lockdep_init_map(&sp->dep_map, name, key, 0);
    + return init_srcu_struct_fields(sp);
    +}
    +EXPORT_SYMBOL_GPL(__init_srcu_struct);
    +
    +#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
    +
    +/**
    + * init_srcu_struct - initialize a sleep-RCU structure
    + * @sp: structure to initialize.
    + *
    + * Must invoke this on a given srcu_struct before passing that srcu_struct
    + * to any other function. Each srcu_struct represents a separate domain
    + * of SRCU protection.
    + */
    +int init_srcu_struct(struct srcu_struct *sp)
    +{
    + return init_srcu_struct_fields(sp);
    +}
    +EXPORT_SYMBOL_GPL(init_srcu_struct);
    +
    +#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
    +
    +/*
    + * Returns approximate total of the readers' ->lock_count[] values for the
    + * rank of per-CPU counters specified by idx.
    + */
    +static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
    +{
    + int cpu;
    + unsigned long sum = 0;
    +
    + for_each_possible_cpu(cpu) {
    + struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
    +
    + sum += READ_ONCE(cpuc->lock_count[idx]);
    + }
    + return sum;
    +}
    +
    +/*
    + * Returns approximate total of the readers' ->unlock_count[] values for the
    + * rank of per-CPU counters specified by idx.
    + */
    +static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
    +{
    + int cpu;
    + unsigned long sum = 0;
    +
    + for_each_possible_cpu(cpu) {
    + struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
    +
    + sum += READ_ONCE(cpuc->unlock_count[idx]);
    + }
    + return sum;
    +}
    +
    +/*
    + * Return true if the number of pre-existing readers is determined to
    + * be zero.
    + */
    +static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
    +{
    + unsigned long unlocks;
    +
    + unlocks = srcu_readers_unlock_idx(sp, idx);
    +
    + /*
    + * Make sure that a lock is always counted if the corresponding
    + * unlock is counted. Needs to be a smp_mb() as the read side may
    + * contain a read from a variable that is written to before the
    + * synchronize_srcu() in the write side. In this case smp_mb()s
    + * A and B act like the store buffering pattern.
    + *
    + * This smp_mb() also pairs with smp_mb() C to prevent accesses
    + * after the synchronize_srcu() from being executed before the
    + * grace period ends.
    + */
    + smp_mb(); /* A */
    +
    + /*
    + * If the locks are the same as the unlocks, then there must have
    + * been no readers on this index at some time in between. This does
    + * not mean that there are no more readers, as one could have read
    + * the current index but not have incremented the lock counter yet.
    + *
    + * Possible bug: There is no guarantee that there haven't been
    + * ULONG_MAX increments of ->lock_count[] since the unlocks were
    + * counted, meaning that this could return true even if there are
    + * still active readers. Since there are no memory barriers around
    + * srcu_flip(), the CPU is not required to increment ->completed
    + * before running srcu_readers_unlock_idx(), which means that there
    + * could be an arbitrarily large number of critical sections that
    + * execute after srcu_readers_unlock_idx() but use the old value
    + * of ->completed.
    + */
    + return srcu_readers_lock_idx(sp, idx) == unlocks;
    +}
    +
    +/**
    + * srcu_readers_active - returns true if there are readers. and false
    + * otherwise
    + * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
    + *
    + * Note that this is not an atomic primitive, and can therefore suffer
    + * severe errors when invoked on an active srcu_struct. That said, it
    + * can be useful as an error check at cleanup time.
    + */
    +static bool srcu_readers_active(struct srcu_struct *sp)
    +{
    + int cpu;
    + unsigned long sum = 0;
    +
    + for_each_possible_cpu(cpu) {
    + struct srcu_array *cpuc = per_cpu_ptr(sp->per_cpu_ref, cpu);
    +
    + sum += READ_ONCE(cpuc->lock_count[0]);
    + sum += READ_ONCE(cpuc->lock_count[1]);
    + sum -= READ_ONCE(cpuc->unlock_count[0]);
    + sum -= READ_ONCE(cpuc->unlock_count[1]);
    + }
    + return sum;
    +}
    +
    +#define SRCU_INTERVAL 1
    +
    +/**
    + * cleanup_srcu_struct - deconstruct a sleep-RCU structure
    + * @sp: structure to clean up.
    + *
    + * Must invoke this after you are finished using a given srcu_struct that
    + * was initialized via init_srcu_struct(), else you leak memory.
    + */
    +void cleanup_srcu_struct(struct srcu_struct *sp)
    +{
    + WARN_ON_ONCE(atomic_read(&sp->srcu_exp_cnt));
    + if (WARN_ON(srcu_readers_active(sp)))
    + return; /* Leakage unless caller handles error. */
    + if (WARN_ON(!rcu_segcblist_empty(&sp->srcu_cblist)))
    + return; /* Leakage unless caller handles error. */
    + flush_delayed_work(&sp->work);
    + if (WARN_ON(rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) != SRCU_STATE_IDLE)) {
    + pr_info("cleanup_srcu_struct: Active srcu_struct %lu CBs %c state: %d\n", rcu_segcblist_n_cbs(&sp->srcu_cblist), ".E"[rcu_segcblist_empty(&sp->srcu_cblist)], rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)));
    + return; /* Caller forgot to stop doing call_srcu()? */
    + }
    + free_percpu(sp->per_cpu_ref);
    + sp->per_cpu_ref = NULL;
    +}
    +EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
    +
    +/*
    + * Counts the new reader in the appropriate per-CPU element of the
    + * srcu_struct. Must be called from process context.
    + * Returns an index that must be passed to the matching srcu_read_unlock().
    + */
    +int __srcu_read_lock(struct srcu_struct *sp)
    +{
    + int idx;
    +
    + idx = READ_ONCE(sp->completed) & 0x1;
    + __this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
    + smp_mb(); /* B */ /* Avoid leaking the critical section. */
    + return idx;
    +}
    +EXPORT_SYMBOL_GPL(__srcu_read_lock);
    +
    +/*
    + * Removes the count for the old reader from the appropriate per-CPU
    + * element of the srcu_struct. Note that this may well be a different
    + * CPU than that which was incremented by the corresponding srcu_read_lock().
    + * Must be called from process context.
    + */
    +void __srcu_read_unlock(struct srcu_struct *sp, int idx)
    +{
    + smp_mb(); /* C */ /* Avoid leaking the critical section. */
    + this_cpu_inc(sp->per_cpu_ref->unlock_count[idx]);
    +}
    +EXPORT_SYMBOL_GPL(__srcu_read_unlock);
    +
    +/*
    + * We use an adaptive strategy for synchronize_srcu() and especially for
    + * synchronize_srcu_expedited(). We spin for a fixed time period
    + * (defined below) to allow SRCU readers to exit their read-side critical
    + * sections. If there are still some readers after a few microseconds,
    + * we repeatedly block for 1-millisecond time periods.
    + */
    +#define SRCU_RETRY_CHECK_DELAY 5
    +
    +/*
    + * Start an SRCU grace period.
    + */
    +static void srcu_gp_start(struct srcu_struct *sp)
    +{
    + int state;
    +
    + rcu_segcblist_accelerate(&sp->srcu_cblist,
    + rcu_seq_snap(&sp->srcu_gp_seq));
    + rcu_seq_start(&sp->srcu_gp_seq);
    + state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
    + WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
    +}
    +
    +/*
    + * Wait until all readers counted by array index idx complete, but
    + * loop an additional time if there is an expedited grace period pending.
    + * The caller must ensure that ->completed is not changed while checking.
    + */
    +static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
    +{
    + for (;;) {
    + if (srcu_readers_active_idx_check(sp, idx))
    + return true;
    + if (--trycount + !!atomic_read(&sp->srcu_exp_cnt) <= 0)
    + return false;
    + udelay(SRCU_RETRY_CHECK_DELAY);
    + }
    +}
    +
    +/*
    + * Increment the ->completed counter so that future SRCU readers will
    + * use the other rank of the ->(un)lock_count[] arrays. This allows
    + * us to wait for pre-existing readers in a starvation-free manner.
    + */
    +static void srcu_flip(struct srcu_struct *sp)
    +{
    + WRITE_ONCE(sp->completed, sp->completed + 1);
    +
    + /*
    + * Ensure that if the updater misses an __srcu_read_unlock()
    + * increment, that task's next __srcu_read_lock() will see the
    + * above counter update. Note that both this memory barrier
    + * and the one in srcu_readers_active_idx_check() provide the
    + * guarantee for __srcu_read_lock().
    + */
    + smp_mb(); /* D */ /* Pairs with C. */
    +}
    +
    +/*
    + * End an SRCU grace period.
    + */
    +static void srcu_gp_end(struct srcu_struct *sp)
    +{
    + rcu_seq_end(&sp->srcu_gp_seq);
    +
    + spin_lock_irq(&sp->queue_lock);
    + rcu_segcblist_advance(&sp->srcu_cblist,
    + rcu_seq_current(&sp->srcu_gp_seq));
    + spin_unlock_irq(&sp->queue_lock);
    +}
    +
    +/*
    + * Enqueue an SRCU callback on the specified srcu_struct structure,
    + * initiating grace-period processing if it is not already running.
    + *
    + * Note that all CPUs must agree that the grace period extended beyond
    + * all pre-existing SRCU read-side critical section. On systems with
    + * more than one CPU, this means that when "func()" is invoked, each CPU
    + * is guaranteed to have executed a full memory barrier since the end of
    + * its last corresponding SRCU read-side critical section whose beginning
    + * preceded the call to call_rcu(). It also means that each CPU executing
    + * an SRCU read-side critical section that continues beyond the start of
    + * "func()" must have executed a memory barrier after the call_rcu()
    + * but before the beginning of that SRCU read-side critical section.
    + * Note that these guarantees include CPUs that are offline, idle, or
    + * executing in user mode, as well as CPUs that are executing in the kernel.
    + *
    + * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
    + * resulting SRCU callback function "func()", then both CPU A and CPU
    + * B are guaranteed to execute a full memory barrier during the time
    + * interval between the call to call_rcu() and the invocation of "func()".
    + * This guarantee applies even if CPU A and CPU B are the same CPU (but
    + * again only if the system has more than one CPU).
    + *
    + * Of course, these guarantees apply only for invocations of call_srcu(),
    + * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
    + * srcu_struct structure.
    + */
    +void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
    + rcu_callback_t func)
    +{
    + unsigned long flags;
    +
    + head->next = NULL;
    + head->func = func;
    + spin_lock_irqsave(&sp->queue_lock, flags);
    + smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */
    + rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
    + if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_IDLE) {
    + srcu_gp_start(sp);
    + queue_delayed_work(system_power_efficient_wq, &sp->work, 0);
    + }
    + spin_unlock_irqrestore(&sp->queue_lock, flags);
    +}
    +EXPORT_SYMBOL_GPL(call_srcu);
    +
    +static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
    +
    +/*
    + * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
    + */
    +static void __synchronize_srcu(struct srcu_struct *sp)
    +{
    + struct rcu_synchronize rcu;
    + struct rcu_head *head = &rcu.head;
    +
    + RCU_LOCKDEP_WARN(lock_is_held(&sp->dep_map) ||
    + lock_is_held(&rcu_bh_lock_map) ||
    + lock_is_held(&rcu_lock_map) ||
    + lock_is_held(&rcu_sched_lock_map),
    + "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
    +
    + if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
    + return;
    + might_sleep();
    + init_completion(&rcu.completion);
    +
    + head->next = NULL;
    + head->func = wakeme_after_rcu;
    + spin_lock_irq(&sp->queue_lock);
    + smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */
    + if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_IDLE) {
    + /* steal the processing owner */
    + rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
    + srcu_gp_start(sp);
    + spin_unlock_irq(&sp->queue_lock);
    + /* give the processing owner to work_struct */
    + srcu_reschedule(sp, 0);
    + } else {
    + rcu_segcblist_enqueue(&sp->srcu_cblist, head, false);
    + spin_unlock_irq(&sp->queue_lock);
    + }
    +
    + wait_for_completion(&rcu.completion);
    + smp_mb(); /* Caller's later accesses after GP. */
    +}
    +
    +/**
    + * synchronize_srcu_expedited - Brute-force SRCU grace period
    + * @sp: srcu_struct with which to synchronize.
    + *
    + * Wait for an SRCU grace period to elapse, but be more aggressive about
    + * spinning rather than blocking when waiting.
    + *
    + * Note that synchronize_srcu_expedited() has the same deadlock and
    + * memory-ordering properties as does synchronize_srcu().
    + */
    +void synchronize_srcu_expedited(struct srcu_struct *sp)
    +{
    + bool do_norm = rcu_gp_is_normal();
    +
    + if (!do_norm) {
    + atomic_inc(&sp->srcu_exp_cnt);
    + smp_mb__after_atomic(); /* increment before GP. */
    + }
    + __synchronize_srcu(sp);
    + if (!do_norm) {
    + smp_mb__before_atomic(); /* GP before decrement. */
    + atomic_dec(&sp->srcu_exp_cnt);
    + }
    +}
    +EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
    +
    +/**
    + * synchronize_srcu - wait for prior SRCU read-side critical-section completion
    + * @sp: srcu_struct with which to synchronize.
    + *
    + * Wait for the count to drain to zero of both indexes. To avoid the
    + * possible starvation of synchronize_srcu(), it waits for the count of
    + * the index=((->completed & 1) ^ 1) to drain to zero at first,
    + * and then flip the completed and wait for the count of the other index.
    + *
    + * Can block; must be called from process context.
    + *
    + * Note that it is illegal to call synchronize_srcu() from the corresponding
    + * SRCU read-side critical section; doing so will result in deadlock.
    + * However, it is perfectly legal to call synchronize_srcu() on one
    + * srcu_struct from some other srcu_struct's read-side critical section,
    + * as long as the resulting graph of srcu_structs is acyclic.
    + *
    + * There are memory-ordering constraints implied by synchronize_srcu().
    + * On systems with more than one CPU, when synchronize_srcu() returns,
    + * each CPU is guaranteed to have executed a full memory barrier since
    + * the end of its last corresponding SRCU-sched read-side critical section
    + * whose beginning preceded the call to synchronize_srcu(). In addition,
    + * each CPU having an SRCU read-side critical section that extends beyond
    + * the return from synchronize_srcu() is guaranteed to have executed a
    + * full memory barrier after the beginning of synchronize_srcu() and before
    + * the beginning of that SRCU read-side critical section. Note that these
    + * guarantees include CPUs that are offline, idle, or executing in user mode,
    + * as well as CPUs that are executing in the kernel.
    + *
    + * Furthermore, if CPU A invoked synchronize_srcu(), which returned
    + * to its caller on CPU B, then both CPU A and CPU B are guaranteed
    + * to have executed a full memory barrier during the execution of
    + * synchronize_srcu(). This guarantee applies even if CPU A and CPU B
    + * are the same CPU, but again only if the system has more than one CPU.
    + *
    + * Of course, these memory-ordering guarantees apply only when
    + * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
    + * passed the same srcu_struct structure.
    + */
    +void synchronize_srcu(struct srcu_struct *sp)
    +{
    + if (rcu_gp_is_expedited())
    + synchronize_srcu_expedited(sp);
    + else
    + __synchronize_srcu(sp);
    +}
    +EXPORT_SYMBOL_GPL(synchronize_srcu);
    +
    +/**
    + * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
    + * @sp: srcu_struct on which to wait for in-flight callbacks.
    + */
    +void srcu_barrier(struct srcu_struct *sp)
    +{
    + synchronize_srcu(sp);
    +}
    +EXPORT_SYMBOL_GPL(srcu_barrier);
    +
    +/**
    + * srcu_batches_completed - return batches completed.
    + * @sp: srcu_struct on which to report batch completion.
    + *
    + * Report the number of batches, correlated with, but not necessarily
    + * precisely the same as, the number of grace periods that have elapsed.
    + */
    +unsigned long srcu_batches_completed(struct srcu_struct *sp)
    +{
    + return sp->completed;
    +}
    +EXPORT_SYMBOL_GPL(srcu_batches_completed);
    +
    +/*
    + * Core SRCU state machine. Advance callbacks from ->batch_check0 to
    + * ->batch_check1 and then to ->batch_done as readers drain.
    + */
    +static void srcu_advance_batches(struct srcu_struct *sp)
    +{
    + int idx;
    +
    + /*
    + * Because readers might be delayed for an extended period after
    + * fetching ->completed for their index, at any point in time there
    + * might well be readers using both idx=0 and idx=1. We therefore
    + * need to wait for readers to clear from both index values before
    + * invoking a callback.
    + *
    + * The load-acquire ensures that we see the accesses performed
    + * by the prior grace period.
    + */
    + idx = rcu_seq_state(smp_load_acquire(&sp->srcu_gp_seq)); /* ^^^ */
    + if (idx == SRCU_STATE_IDLE) {
    + spin_lock_irq(&sp->queue_lock);
    + if (rcu_segcblist_empty(&sp->srcu_cblist)) {
    + spin_unlock_irq(&sp->queue_lock);
    + return;
    + }
    + idx = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
    + if (idx == SRCU_STATE_IDLE)
    + srcu_gp_start(sp);
    + spin_unlock_irq(&sp->queue_lock);
    + if (idx != SRCU_STATE_IDLE)
    + return; /* Someone else started the grace period. */
    + }
    +
    + if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
    + idx = 1 ^ (sp->completed & 1);
    + if (!try_check_zero(sp, idx, 1))
    + return; /* readers present, retry later. */
    + srcu_flip(sp);
    + rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2);
    + }
    +
    + if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {
    +
    + /*
    + * SRCU read-side critical sections are normally short,
    + * so check at least twice in quick succession after a flip.
    + */
    + idx = 1 ^ (sp->completed & 1);
    + if (!try_check_zero(sp, idx, 2))
    + return; /* readers present, retry after later. */
    + srcu_gp_end(sp);
    + }
    +}
    +
    +/*
    + * Invoke a limited number of SRCU callbacks that have passed through
    + * their grace period. If there are more to do, SRCU will reschedule
    + * the workqueue. Note that needed memory barriers have been executed
    + * in this task's context by srcu_readers_active_idx_check().
    + */
    +static void srcu_invoke_callbacks(struct srcu_struct *sp)
    +{
    + struct rcu_cblist ready_cbs;
    + struct rcu_head *rhp;
    +
    + spin_lock_irq(&sp->queue_lock);
    + if (!rcu_segcblist_ready_cbs(&sp->srcu_cblist)) {
    + spin_unlock_irq(&sp->queue_lock);
    + return;
    + }
    + rcu_cblist_init(&ready_cbs);
    + rcu_segcblist_extract_done_cbs(&sp->srcu_cblist, &ready_cbs);
    + spin_unlock_irq(&sp->queue_lock);
    + rhp = rcu_cblist_dequeue(&ready_cbs);
    + for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
    + local_bh_disable();
    + rhp->func(rhp);
    + local_bh_enable();
    + }
    + spin_lock_irq(&sp->queue_lock);
    + rcu_segcblist_insert_count(&sp->srcu_cblist, &ready_cbs);
    + spin_unlock_irq(&sp->queue_lock);
    +}
    +
    +/*
    + * Finished one round of SRCU grace period. Start another if there are
    + * more SRCU callbacks queued, otherwise put SRCU into not-running state.
    + */
    +static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay)
    +{
    + bool pending = true;
    + int state;
    +
    + if (rcu_segcblist_empty(&sp->srcu_cblist)) {
    + spin_lock_irq(&sp->queue_lock);
    + state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
    + if (rcu_segcblist_empty(&sp->srcu_cblist) &&
    + state == SRCU_STATE_IDLE)
    + pending = false;
    + spin_unlock_irq(&sp->queue_lock);
    + }
    +
    + if (pending)
    + queue_delayed_work(system_power_efficient_wq, &sp->work, delay);
    +}
    +
    +/*
    + * This is the work-queue function that handles SRCU grace periods.
    + */
    +void process_srcu(struct work_struct *work)
    +{
    + struct srcu_struct *sp;
    +
    + sp = container_of(work, struct srcu_struct, work.work);
    +
    + srcu_advance_batches(sp);
    + srcu_invoke_callbacks(sp);
    + srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
    +}
    +EXPORT_SYMBOL_GPL(process_srcu);
    --
    2.5.2
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      Last update: 2017-04-19 19:08    [W:3.264 / U:0.256 seconds]
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