Messages in this thread | | | Date | Thu, 1 Apr 2021 18:12:41 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 3/3] srcu: Fix broken node geometry after early ssp init |
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On Fri, Apr 02, 2021 at 01:47:04AM +0200, Frederic Weisbecker wrote: > 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>
Again, good catch and thank you!
One question below.
Thanx, Paul
> --- > 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);
Why not just pull all of the pre-initialization callbacks onto a local list (re-initialization the rcu_segcblist structures if necessary), then iterate through that list re-invoking call_srcu() on each? There shouldn't be that many pre-initialization call_srcu() invocations, and if someday there are, it would not be hard to make __call_srcu() take lists of callbacks and a count instead of a single callback, correct?
Or am I once again missing something basic?
Thanx, Paul
> + } > + > + /* 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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