perf: Fix put_event() ctx lock

So what I suspect; but I'm in zombie mode today it seems; is that while
I initially thought that it was impossible for ctx to change when
refcount dropped to 0, I now suspect its possible.

Note that until perf_remove_from_context() the event is still active and
visible on the lists. So a concurrent sys_perf_event_open() from another
task into this task can race.

Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Stephane Eranian <eranian@gmail.com>
Cc: mark.rutland@arm.com
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20150129134434.GB26304@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 142dbab..f773fa1 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -947,7 +947,8 @@ static void put_ctx(struct perf_event_context *ctx)
  *         perf_event::mmap_mutex
  *         mmap_sem
  */
-static struct perf_event_context *perf_event_ctx_lock(struct perf_event *event)
+static struct perf_event_context *
+perf_event_ctx_lock_nested(struct perf_event *event, int nesting)
 {
        struct perf_event_context *ctx;
 
@@ -960,7 +961,7 @@ again:
        }
        rcu_read_unlock();
 
-       mutex_lock(&ctx->mutex);
+       mutex_lock_nested(&ctx->mutex, nesting);
        if (event->ctx != ctx) {
                mutex_unlock(&ctx->mutex);
                put_ctx(ctx);
@@ -970,6 +971,12 @@ again:
        return ctx;
 }
 
+static inline struct perf_event_context *
+perf_event_ctx_lock(struct perf_event *event)
+{
+       return perf_event_ctx_lock_nested(event, 0);
+}
+
 static void perf_event_ctx_unlock(struct perf_event *event,
                                  struct perf_event_context *ctx)
 {
@@ -3572,7 +3579,7 @@ static void perf_remove_from_owner(struct perf_event *event)
  */
 static void put_event(struct perf_event *event)
 {
-       struct perf_event_context *ctx = event->ctx;
+       struct perf_event_context *ctx;
 
        if (!atomic_long_dec_and_test(&event->refcount))
                return;
@@ -3580,7 +3587,6 @@ static void put_event(struct perf_event *event)
        if (!is_kernel_event(event))
                perf_remove_from_owner(event);
 
-       WARN_ON_ONCE(ctx->parent_ctx);
        /*
         * There are two ways this annotation is useful:
         *
@@ -3593,7 +3599,8 @@ static void put_event(struct perf_event *event)
         *     the last filedesc died, so there is no possibility
         *     to trigger the AB-BA case.
         */
-       mutex_lock_nested(&ctx->mutex, SINGLE_DEPTH_NESTING);
+       ctx = perf_event_ctx_lock_nested(event, SINGLE_DEPTH_NESTING);
+       WARN_ON_ONCE(ctx->parent_ctx);
        perf_remove_from_context(event, true);
        mutex_unlock(&ctx->mutex);