6.6-stable review patch. If anyone has any objections, please let me know.
------------------
From: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
commit 74751ef5c1912ebd3e65c3b65f45587e05ce5d36 upstream.
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: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/events/core.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5353,6 +5353,7 @@ int perf_event_release_kernel(struct per
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
@@ -5393,11 +5394,23 @@ again:
* 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);
