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charset="utf-8" Content-Transfer-Encoding: quoted-printable The following commit has been merged into the perf/core branch of tip: Commit-ID: 2839f393c69456bc356738e521b2e70b82977f46 Gitweb: https://git.kernel.org/tip/2839f393c69456bc356738e521b2e70b8= 2977f46 Author: Frederic Weisbecker AuthorDate: Wed, 09 Apr 2025 15:01:12 +02:00 Committer: Ingo Molnar CommitterDate: Thu, 17 Apr 2025 14:21:15 +02:00 perf/core: Fix put_ctx() ordering So there are three situations: * If perf_event_free_task() has removed all the children from the parent li= st before perf_event_release_kernel() got a chance to even iterate them, then it's all good as there is no get_ctx() pending. * If perf_event_release_kernel() iterates a child event, but it gets freed meanwhile by perf_event_free_task() while the mutexes are temporarily unlocked, it's all good because while locking again the ctx mutex, perf_event_release_kernel() observes TASK_TOMBSTONE. * But if perf_event_release_kernel() frees the child event before perf_event_free_task() got a chance, we may face this scenario: perf_event_release_kernel() perf_event= _free_task() -------------------------- ----------= -------------- mutex_lock(&event->child_mutex) get_ctx(child->ctx) mutex_unlock(&event->child_mutex) mutex_lock(ctx->mutex) mutex_lock(&event->child_mutex) perf_remove_from_context(child) mutex_unlock(&event->child_mutex) mutex_unlock(ctx->mutex) // This lo= ck acquires ctx->refcount =3D=3D 2 // visibil= ity mutex_lock= (ctx->mutex) ctx->task = =3D TASK_TOMBSTONE mutex_unlo= ck(ctx->mutex) wait_var_e= vent() // ent= ers prepare_to_wait() since // ctx= ->refcount =3D=3D 2 // is = guaranteed to be seen set_cu= rrent_state(TASK_INTERRUPTIBLE) smp_mb= () if (ct= x->refcount !=3D 1) sc= hedule() put_ctx() // NOT fully ordered! Only RELEASE semantics refcount_dec_and_test() atomic_fetch_sub_release() // So TASK_TOMBSTONE is not guaranteed to be seen if (ctx->task =3D=3D TASK_TOMBSTONE) wake_up_var() Basically it's a broken store buffer: perf_event_release_kernel() perf_event= _free_task() -------------------------- ----------= -------------- ctx->task =3D TASK_TOMBSTONE smp_stor= e_release(&ctx->refcount, ctx->refcount - 1) smp_mb() READ_ONCE(ctx->refcount) READ_ONCE(= ctx->task) So we need a smp_mb__after_atomic() before looking at ctx->task. Fixes: 59f3aa4a3ee2 ("perf: Simplify perf_event_free_task() wait") Signed-off-by: Frederic Weisbecker Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Link: https://lkml.kernel.org/r/Z_ZvmEhjkAhplCBE@localhost.localdomain --- kernel/events/core.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index e4d7a0c..1a19df9 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -1271,9 +1271,10 @@ static void put_ctx(struct perf_event_context *ctx) if (ctx->task && ctx->task !=3D TASK_TOMBSTONE) put_task_struct(ctx->task); call_rcu(&ctx->rcu_head, free_ctx); - } else if (ctx->task =3D=3D TASK_TOMBSTONE) { - smp_mb(); /* pairs with wait_var_event() */ - wake_up_var(&ctx->refcount); + } else { + smp_mb__after_atomic(); /* pairs with wait_var_event() */ + if (ctx->task =3D=3D TASK_TOMBSTONE) + wake_up_var(&ctx->refcount); } }