perf: Fix race in removing an event

When removing a (sibling) event we do:

	raw_spin_lock_irq(&ctx->lock);
	perf_group_detach(event);
	raw_spin_unlock_irq(&ctx->lock);

	<hole>

	perf_remove_from_context(event);
		raw_spin_lock_irq(&ctx->lock);
		...
		raw_spin_unlock_irq(&ctx->lock);

Now, assuming the event is a sibling, it will be 'unreachable' for
things like ctx_sched_out() because that iterates the
groups->siblings, and we just unhooked the sibling.

So, if during <hole> we get ctx_sched_out(), it will miss the event
and not call event_sched_out() on it, leaving it programmed on the
PMU.

The subsequent perf_remove_from_context() call will find the ctx is
inactive and only call list_del_event() to remove the event from all
other lists.

Hereafter we can proceed to free the event; while still programmed!

Close this hole by moving perf_group_detach() inside the same
ctx->lock region(s) perf_remove_from_context() has.

The condition on inherited events only in __perf_event_exit_task() is
likely complete crap because non-inherited events are part of groups
too and we're tearing down just the same. But leave that for another
patch.

Most-likely-Fixes: e03a9a55b4e ("perf: Change close() semantics for group events")
Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Tested-by: Vince Weaver <vincent.weaver@maine.edu>
Much-staring-at-traces-by: Vince Weaver <vincent.weaver@maine.edu>
Much-staring-at-traces-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140505093124.GN17778@laptop.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 26 insertions, 21 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f83a71a3e46d..ea899e2b5593 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1443,6 +1443,11 @@ group_sched_out(struct perf_event *group_event,
                 cpuctx->exclusive = 0;
 }
 
+struct remove_event {
+        struct perf_event *event;
+        bool detach_group;
+};
+
 /*
  * Cross CPU call to remove a performance event
  *
@@ -1451,12 +1456,15 @@ group_sched_out(struct perf_event *group_event,
  */
 static int __perf_remove_from_context(void *info)
 {
-        struct perf_event *event = info;
+        struct remove_event *re = info;
+        struct perf_event *event = re->event;
         struct perf_event_context *ctx = event->ctx;
         struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
 
         raw_spin_lock(&ctx->lock);
         event_sched_out(event, cpuctx, ctx);
+        if (re->detach_group)
+                perf_group_detach(event);
         list_del_event(event, ctx);
         if (!ctx->nr_events && cpuctx->task_ctx == ctx) {
                 ctx->is_active = 0;
@@ -1481,10 +1489,14 @@ static int __perf_remove_from_context(void *info)
  * When called from perf_event_exit_task, it's OK because the
  * context has been detached from its task.
  */
-static void perf_remove_from_context(struct perf_event *event)
+static void perf_remove_from_context(struct perf_event *event, bool detach_group)
 {
         struct perf_event_context *ctx = event->ctx;
         struct task_struct *task = ctx->task;
+        struct remove_event re = {
+                .event = event,
+                .detach_group = detach_group,
+        };
 
         lockdep_assert_held(&ctx->mutex);
 
@@ -1493,12 +1505,12 @@ static void perf_remove_from_context(struct perf_event *event)
                  * Per cpu events are removed via an smp call and
                  * the removal is always successful.
                  */
-                cpu_function_call(event->cpu, __perf_remove_from_context, event);
+                cpu_function_call(event->cpu, __perf_remove_from_context, &re);
                 return;
         }
 
 retry:
-        if (!task_function_call(task, __perf_remove_from_context, event))
+        if (!task_function_call(task, __perf_remove_from_context, &re))
                 return;
 
         raw_spin_lock_irq(&ctx->lock);
@@ -1515,6 +1527,8 @@ retry:
          * Since the task isn't running, its safe to remove the event, us
          * holding the ctx->lock ensures the task won't get scheduled in.
          */
+        if (detach_group)
+                perf_group_detach(event);
         list_del_event(event, ctx);
         raw_spin_unlock_irq(&ctx->lock);
 }
@@ -3281,10 +3295,7 @@ int perf_event_release_kernel(struct perf_event *event)
          *     to trigger the AB-BA case.
          */
         mutex_lock_nested(&ctx->mutex, SINGLE_DEPTH_NESTING);
-        raw_spin_lock_irq(&ctx->lock);
-        perf_group_detach(event);
-        raw_spin_unlock_irq(&ctx->lock);
-        perf_remove_from_context(event);
+        perf_remove_from_context(event, true);
         mutex_unlock(&ctx->mutex);
 
         free_event(event);
@@ -7165,7 +7176,7 @@ SYSCALL_DEFINE5(perf_event_open,
                 struct perf_event_context *gctx = group_leader->ctx;
 
                 mutex_lock(&gctx->mutex);
-                perf_remove_from_context(group_leader);
+                perf_remove_from_context(group_leader, false);
 
                 /*
                  * Removing from the context ends up with disabled
@@ -7175,7 +7186,7 @@ SYSCALL_DEFINE5(perf_event_open,
                 perf_event__state_init(group_leader);
                 list_for_each_entry(sibling, &group_leader->sibling_list,
                                     group_entry) {
-                        perf_remove_from_context(sibling);
+                        perf_remove_from_context(sibling, false);
                         perf_event__state_init(sibling);
                         put_ctx(gctx);
                 }
@@ -7305,7 +7316,7 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
         mutex_lock(&src_ctx->mutex);
         list_for_each_entry_safe(event, tmp, &src_ctx->event_list,
                                  event_entry) {
-                perf_remove_from_context(event);
+                perf_remove_from_context(event, false);
                 unaccount_event_cpu(event, src_cpu);
                 put_ctx(src_ctx);
                 list_add(&event->migrate_entry, &events);
@@ -7367,13 +7378,7 @@ __perf_event_exit_task(struct perf_event *child_event,
                          struct perf_event_context *child_ctx,
                          struct task_struct *child)
 {
-        if (child_event->parent) {
-                raw_spin_lock_irq(&child_ctx->lock);
-                perf_group_detach(child_event);
-                raw_spin_unlock_irq(&child_ctx->lock);
-        }
-
-        perf_remove_from_context(child_event);
+        perf_remove_from_context(child_event, !!child_event->parent);
 
         /*
          * It can happen that the parent exits first, and has events
@@ -7857,14 +7862,14 @@ static void perf_pmu_rotate_stop(struct pmu *pmu)
 
 static void __perf_event_exit_context(void *__info)
 {
+        struct remove_event re = { .detach_group = false };
         struct perf_event_context *ctx = __info;
-        struct perf_event *event;
 
         perf_pmu_rotate_stop(ctx->pmu);
 
         rcu_read_lock();
-        list_for_each_entry_rcu(event, &ctx->event_list, event_entry)
-                __perf_remove_from_context(event);
+        list_for_each_entry_rcu(re.event, &ctx->event_list, event_entry)
+                __perf_remove_from_context(&re);
         rcu_read_unlock();
 }