This is a note to let you know that I've just added the patch titled
perf/core: Fix missing wakeup when waiting for context reference
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
perf-core-fix-missing-wakeup-when-waiting-for-contex.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 5763c50cd8d4541e5927382ae26935e23c10e7d9
Author: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
Date: Mon May 13 10:39:48 2024 +0000
perf/core: Fix missing wakeup when waiting for context reference
[ Upstream commit 74751ef5c1912ebd3e65c3b65f45587e05ce5d36 ]
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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20240513103948.33570-1-haifeng.xu@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/events/core.c b/kernel/events/core.c
index e79cd0fd1d2b7..80d9c8fcc30a6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5276,6 +5276,7 @@ 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
@@ -5316,11 +5317,23 @@ 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);
