trace: Fix epoll hang when we race with new entries

[platform/adaptation/renesas_rcar/renesas_kernel.git] / kernel / trace / ring_buffer.c

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index a53f1bb..773aba8 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -626,8 +626,22 @@ int ring_buffer_poll_wait(struct ring_buffer *buffer, int cpu,
                work = &cpu_buffer->irq_work;
        }
 
-       work->waiters_pending = true;
        poll_wait(filp, &work->waiters, poll_table);
+       work->waiters_pending = true;
+       /*
+        * There's a tight race between setting the waiters_pending and
+        * checking if the ring buffer is empty.  Once the waiters_pending bit
+        * is set, the next event will wake the task up, but we can get stuck
+        * if there's only a single event in.
+        *
+        * FIXME: Ideally, we need a memory barrier on the writer side as well,
+        * but adding a memory barrier to all events will cause too much of a
+        * performance hit in the fast path.  We only need a memory barrier when
+        * the buffer goes from empty to having content.  But as this race is
+        * extremely small, and it's not a problem if another event comes in, we
+        * will fix it later.
+        */
+       smp_mb();
 
        if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) ||
            (cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, cpu)))