From 189613e5ef510bc6f58feeb023b57d71c1970a4c Mon Sep 17 00:00:00 2001 From: Cong Wang Date: Tue, 2 Sep 2014 15:27:20 -0700 Subject: [PATCH] perf: Fix a race condition in perf_remove_from_context() commit 3577af70a2ce4853d58e57d832e687d739281479 upstream. We saw a kernel soft lockup in perf_remove_from_context(), it looks like the `perf` process, when exiting, could not go out of the retry loop. Meanwhile, the target process was forking a child. So either the target process should execute the smp function call to deactive the event (if it was running) or it should do a context switch which deactives the event. It seems we optimize out a context switch in perf_event_context_sched_out(), and what's more important, we still test an obsolete task pointer when retrying, so no one actually would deactive that event in this situation. Fix it directly by reloading the task pointer in perf_remove_from_context(). This should cure the above soft lockup. Signed-off-by: Cong Wang Signed-off-by: Cong Wang Signed-off-by: Peter Zijlstra Cc: Paul Mackerras Cc: Arnaldo Carvalho de Melo Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1409696840-843-1-git-send-email-xiyou.wangcong@gmail.com Signed-off-by: Ingo Molnar Signed-off-by: Ben Hutchings --- kernel/events/core.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/kernel/events/core.c b/kernel/events/core.c index 14c111cf5047..a11d537f444d 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -1683,6 +1683,16 @@ retry: */ if (ctx->is_active) { raw_spin_unlock_irq(&ctx->lock); + /* + * Reload the task pointer, it might have been changed by + * a concurrent perf_event_context_sched_out(). + */ + task = ctx->task; + /* + * Reload the task pointer, it might have been changed by + * a concurrent perf_event_context_sched_out(). + */ + task = ctx->task; goto retry; } -- 2.39.2