Messages in this thread Patch in this message | | | Date | Thu, 18 Apr 2024 12:03:00 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v2] perf/core: Fix missing wakeup when waiting for context reference |
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On Thu, Apr 18, 2024 at 08:03:56AM +0000, Haifeng Xu wrote: > In our production environment, we found many hung tasks which are > blocked for more than 18 hours. Their call traces are like this: > > [346278.191038] __schedule+0x2d8/0x890 > [346278.191046] schedule+0x4e/0xb0 > [346278.191049] perf_event_free_task+0x220/0x270 > [346278.191056] ? init_wait_var_entry+0x50/0x50 > [346278.191060] copy_process+0x663/0x18d0 > [346278.191068] kernel_clone+0x9d/0x3d0 > [346278.191072] __do_sys_clone+0x5d/0x80 > [346278.191076] __x64_sys_clone+0x25/0x30 > [346278.191079] do_syscall_64+0x5c/0xc0 > [346278.191083] ? syscall_exit_to_user_mode+0x27/0x50 > [346278.191086] ? do_syscall_64+0x69/0xc0 > [346278.191088] ? irqentry_exit_to_user_mode+0x9/0x20 > [346278.191092] ? irqentry_exit+0x19/0x30 > [346278.191095] ? exc_page_fault+0x89/0x160 > [346278.191097] ? asm_exc_page_fault+0x8/0x30 > [346278.191102] entry_SYSCALL_64_after_hwframe+0x44/0xae > > The task was waiting for the refcount become to 1, but from the vmcore, > we found the refcount has already been 1. It seems that the task didn't > get woken up by perf_event_release_kernel() and got stuck forever. The > below scenario may cause the problem. > > Thread A Thread B > ... ... > perf_event_free_task perf_event_release_kernel > ... > acquire event->child_mutex > ... > get_ctx > ... release event->child_mutex > acquire ctx->mutex > ... > perf_free_event (acquire/release event->child_mutex) > ... > release ctx->mutex > wait_var_event > acquire ctx->mutex > acquire event->child_mutex > # move existing events to free_list > release event->child_mutex > release ctx->mutex > put_ctx > ... ... > > In this case, all events of the ctx have been freed, so we couldn't > find the ctx in free_list and Thread A will miss the wakeup. It's thus > necessary to add a wakeup after dropping the reference. > > Fixes: 1cf8dfe8a661 ("perf/core: Fix race between close() and fork()") > Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Hint: always ask before putting someone else's Signed-off-by tag ;-) And anyway you don't need it here.
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> --- > Changes since v1 > - Add the fixed tag. > - Simplify v1's patch. (Frederic) > --- > kernel/events/core.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/kernel/events/core.c b/kernel/events/core.c > index 4f0c45ab8d7d..15c35070db6a 100644 > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -5340,6 +5340,7 @@ int perf_event_release_kernel(struct perf_event *event) > again: > mutex_lock(&event->child_mutex); > list_for_each_entry(child, &event->child_list, child_list) { > + void *var = NULL; > > /* > * Cannot change, child events are not migrated, see the > @@ -5380,11 +5381,23 @@ int perf_event_release_kernel(struct perf_event *event) > * this can't be the last reference. > */ > put_event(event); > + } else { > + var = &ctx->refcount; > } > > mutex_unlock(&event->child_mutex); > mutex_unlock(&ctx->mutex); > put_ctx(ctx); > + > + if (var) { > + /* > + * If perf_event_free_task() has deleted all events from the > + * ctx while the child_mutex got released above, make sure to > + * notify about the preceding put_ctx(). > + */ > + smp_mb(); /* pairs with wait_var_event() */ > + wake_up_var(var); > + } > goto again; > } > mutex_unlock(&event->child_mutex); > -- > 2.25.1 >
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