Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [PATCH 3/3] srcu: Fix broken node geometry after early ssp init | Date | Fri, 2 Apr 2021 01:47:04 +0200 |
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An ssp initialized before rcu_init_geometry() will have its snp hierarchy based on CONFIG_NR_CPUS.
Once rcu_init_geometry() is called, the nodes distribution is shrinked and optimized toward meeting the actual possible number of CPUs detected on boot.
Later on, the ssp that was initialized before rcu_init_geometry() is confused and sometimes refers to its initial CONFIG_NR_CPUS based node hierarchy, sometimes to the new num_possible_cpus() based one instead. For example each of its sdp->mynode remain backward and refer to the early node leaves that may not exist anymore. On the other hand the srcu_for_each_node_breadth_first() refers to the new node hierarchy.
There are at least two bad possible outcomes to this:
1) a) A callback enqueued early on an sdp is recorded pending on sdp->mynode->srcu_data_have_cbs in srcu_funnel_gp_start() with sdp->mynode pointing to a deep leaf (say 3 levels).
b) The grace period ends after rcu_init_geometry() which shrinks the nodes level to a single one. srcu_gp_end() walks through the new snp hierarchy without ever reaching the old leaves so the callback is never executed.
This is easily reproduced on an 8 CPUs machine with CONFIG_NR_CPUS >= 32 and "rcupdate.rcu_self_test=1". The srcu_barrier() after early tests verification never completes and the boot hangs:
[ 5413.141029] INFO: task swapper/0:1 blocked for more than 4915 seconds. [ 5413.147564] Not tainted 5.12.0-rc4+ #28 [ 5413.151927] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 5413.159753] task:swapper/0 state:D stack: 0 pid: 1 ppid: 0 flags:0x00004000 [ 5413.168099] Call Trace: [ 5413.170555] __schedule+0x36c/0x930 [ 5413.174057] ? wait_for_completion+0x88/0x110 [ 5413.178423] schedule+0x46/0xf0 [ 5413.181575] schedule_timeout+0x284/0x380 [ 5413.185591] ? wait_for_completion+0x88/0x110 [ 5413.189957] ? mark_held_locks+0x61/0x80 [ 5413.193882] ? mark_held_locks+0x61/0x80 [ 5413.197809] ? _raw_spin_unlock_irq+0x24/0x50 [ 5413.202173] ? wait_for_completion+0x88/0x110 [ 5413.206535] wait_for_completion+0xb4/0x110 [ 5413.210724] ? srcu_torture_stats_print+0x110/0x110 [ 5413.215610] srcu_barrier+0x187/0x200 [ 5413.219277] ? rcu_tasks_verify_self_tests+0x50/0x50 [ 5413.224244] ? rdinit_setup+0x2b/0x2b [ 5413.227907] rcu_verify_early_boot_tests+0x2d/0x40 [ 5413.232700] do_one_initcall+0x63/0x310 [ 5413.236541] ? rdinit_setup+0x2b/0x2b [ 5413.240207] ? rcu_read_lock_sched_held+0x52/0x80 [ 5413.244912] kernel_init_freeable+0x253/0x28f [ 5413.249273] ? rest_init+0x250/0x250 [ 5413.252846] kernel_init+0xa/0x110 [ 5413.256257] ret_from_fork+0x22/0x30
2) An ssp that gets initialized before rcu_init_geometry() and used afterward will always have stale rdp->mynode references, resulting in callbacks to be missed in srcu_gp_end(), just like in the previous scenario.
Solve this with fixing the node tree layout of early initialized ssp once rcu_init_geometry() is done. Unfortunately this involves a new field into struct srcu_struct to postpone the ssp hierarchy rebuild.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Cc: Boqun Feng <boqun.feng@gmail.com> Cc: Lai Jiangshan <jiangshanlai@gmail.com> Cc: Neeraj Upadhyay <neeraju@codeaurora.org> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Joel Fernandes <joel@joelfernandes.org> Cc: Uladzislau Rezki <urezki@gmail.com> --- include/linux/srcutree.h | 1 + kernel/rcu/srcutree.c | 68 +++++++++++++++++++++++++++++++++++----- 2 files changed, 62 insertions(+), 7 deletions(-)
diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h index 9cfcc8a756ae..4339e5794a72 100644 --- a/include/linux/srcutree.h +++ b/include/linux/srcutree.h @@ -85,6 +85,7 @@ struct srcu_struct { #ifdef CONFIG_DEBUG_LOCK_ALLOC struct lockdep_map dep_map; #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ + struct list_head early_init; }; /* Values for state variable (bottom bits of ->srcu_gp_seq). */ diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 10e681ea7051..285f0c053754 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -39,7 +39,7 @@ static ulong counter_wrap_check = (ULONG_MAX >> 2); module_param(counter_wrap_check, ulong, 0444); /* Early-boot callback-management, so early that no lock is required! */ -static LIST_HEAD(srcu_boot_list); +static LIST_HEAD(srcu_boot_queue_list); static bool __read_mostly srcu_init_done; static void srcu_invoke_callbacks(struct work_struct *work); @@ -133,7 +133,9 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp) for_each_possible_cpu(cpu) { sdp = per_cpu_ptr(ssp->sda, cpu); spin_lock_init(&ACCESS_PRIVATE(sdp, lock)); - rcu_segcblist_init(&sdp->srcu_cblist); + /* Preserve the queue in case of hierarchy rebuild */ + if (!rcu_segcblist_is_enabled(&sdp->srcu_cblist)) + rcu_segcblist_init(&sdp->srcu_cblist); sdp->srcu_cblist_invoking = false; sdp->srcu_gp_seq_needed = ssp->srcu_gp_seq; sdp->srcu_gp_seq_needed_exp = ssp->srcu_gp_seq; @@ -151,6 +153,8 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp) } } +static LIST_HEAD(srcu_early_init_list); + /* * Initialize non-compile-time initialized fields, including the * associated srcu_node and srcu_data structures. The is_static @@ -174,6 +178,8 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static) init_srcu_struct_nodes(ssp); ssp->srcu_gp_seq_needed_exp = 0; ssp->srcu_last_gp_end = ktime_get_mono_fast_ns(); + if (!srcu_init_done) + list_add_tail(&ssp->early_init, &srcu_early_init_list); smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */ return 0; } @@ -679,7 +685,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, queue_delayed_work(rcu_gp_wq, &ssp->work, srcu_get_delay(ssp)); else if (list_empty(&ssp->work.work.entry)) - list_add(&ssp->work.work.entry, &srcu_boot_list); + list_add(&ssp->work.work.entry, &srcu_boot_queue_list); } spin_unlock_irqrestore_rcu_node(ssp, flags); } @@ -1380,14 +1386,62 @@ static int __init srcu_bootup_announce(void) } early_initcall(srcu_bootup_announce); +static void __init restore_srcu_sdp(struct srcu_data *sdp, + struct srcu_data *old_sdp, + struct srcu_node *old_mynode) +{ + int i; + struct srcu_node *mynode = sdp->mynode; + struct srcu_node *snp = mynode; + + /* No need to lock the nodes at this stage. We are on early boot */ + for (snp = mynode; snp != NULL; snp = snp->srcu_parent) { + for (i = 0; i < ARRAY_SIZE(old_mynode->srcu_have_cbs); i++) { + snp->srcu_have_cbs[i] = old_mynode->srcu_have_cbs[i]; + + if (snp != mynode) + continue; + + if (old_mynode->srcu_data_have_cbs[i] == old_sdp->grpmask) { + snp->srcu_data_have_cbs[i] = sdp->grpmask; + } else if (!old_mynode->srcu_data_have_cbs[i]) { + snp->srcu_data_have_cbs[i] = 0; + } else { + /* Don't expect other CPUs to have callbacks that early */ + WARN_ON_ONCE(1); + } + } + snp->srcu_gp_seq_needed_exp = old_mynode->srcu_gp_seq_needed_exp; + } + + sdp->srcu_gp_seq_needed = old_sdp->srcu_gp_seq_needed; + sdp->srcu_gp_seq_needed_exp = old_sdp->srcu_gp_seq_needed_exp; +} + void __init srcu_init(void) { - struct srcu_struct *ssp; + static struct srcu_data __initdata old_sdp; + static struct srcu_node __initdata old_mynode; + struct srcu_struct *ssp, *tmp; srcu_init_done = true; - while (!list_empty(&srcu_boot_list)) { - ssp = list_first_entry(&srcu_boot_list, struct srcu_struct, - work.work.entry); + /* + * ssp's that got initialized before rcu_init_geometry() have a stale + * node hierarchy. Rebuild their node trees and propagate states from + * pruned leaf nodes. + */ + list_for_each_entry_safe(ssp, tmp, &srcu_early_init_list, early_init) { + struct srcu_data *sdp = this_cpu_ptr(ssp->sda); + + list_del(&ssp->early_init); + old_sdp = *sdp; + old_mynode = *sdp->mynode; + init_srcu_struct_nodes(ssp); + restore_srcu_sdp(sdp, &old_sdp, &old_mynode); + } + + /* Handle works that had to wait before workqueues get created */ + list_for_each_entry_safe(ssp, tmp, &srcu_boot_queue_list, work.work.entry) { list_del_init(&ssp->work.work.entry); queue_work(rcu_gp_wq, &ssp->work.work); } -- 2.25.1
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