Re: BUG: using smp_processor_id() in preemptible [00000000] code: asm/8267

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2013-03-27 at 10:49 +0100, Borislav Petkov wrote:
> Ok, just for my own understanding: how do the events on the
> ->task_ctx->event_list relate to the current cpu in this path? I mean,
> we're on the task exit path here so is it possible to be rescheduled
> somewhere else and the check in event_filter_match to become
> meaningless?

Events can be per-cpu, so what could happen is that we'd send the exit
notification to a cpu we're not actually running on (anymore).

Furthermore, since we evaluate smp_processor_id() for every
event_filter_match(), it is possible we'd send the notification to
multiple events if our task migrates just right.

Also, I suspect we want something like preempt_disable_notrace() to be
sure we don't recurse or something daft like that... but I'm not
entirely sure.

---
 kernel/events/core.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7b4a55d..0097d81 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4444,19 +4444,22 @@ static void perf_event_task_event(struct perf_task_event *task_event)
 		perf_event_task_ctx(&cpuctx->ctx, task_event);
 
 		ctx = task_event->task_ctx;
-		if (!ctx) {
-			ctxn = pmu->task_ctx_nr;
-			if (ctxn < 0)
-				goto next;
-			ctx = rcu_dereference(current->perf_event_ctxp[ctxn]);
-			if (ctx)
-				perf_event_task_ctx(ctx, task_event);
-		}
+		if (ctx)
+			goto next;
+		ctxn = pmu->task_ctx_nr;
+		if (ctxn < 0)
+			goto next;
+		ctx = rcu_dereference(current->perf_event_ctxp[ctxn]);
+		if (ctx)
+			perf_event_task_ctx(ctx, task_event);
 next:
 		put_cpu_ptr(pmu->pmu_cpu_context);
 	}
-	if (task_event->task_ctx)
+	if (task_event->task_ctx) {
+		preempt_disable_notrace();
 		perf_event_task_ctx(task_event->task_ctx, task_event);
+		preempt_enable_notrace();
+	}
 
 	rcu_read_unlock();
 }