Merge tag 'imx-cleanup-3.18' of git://git.kernel.org/pub/scm/linux/kernel/git/shawnguo/linux into next/cleanup

Merge "ARM: imx: cleanup for 3.18" from Shawn Guo:

The i.MX cleanup for 3.18:
 - Reomve a few i.MX27 and i.MX1 board files
 - Remove imx_scu_standby_enable() since core code handles scu
   standby now
 - Remove unnecessary iomux declaration
 - Remove useless sound card property from vf610-twr dts

* tag 'imx-cleanup-3.18' of git://git.kernel.org/pub/scm/linux/kernel/git/shawnguo/linux:
  ARM: imx: Remove mach-mxt_td60 board file
  ARM: i.MX: Remove i.MX1 ADS board support
  ARM: dts: vf610-twr: remove useless property for sound card.
  ARM: imx: remove imx_scu_standby_enable()
  ARM: i.MX: Remove Phytec i.MX27 PCM038/PCM970 board files
  ARM: i.MX: Remove mach-cpuimx27sd board file
  ARM: imx: iomux: Do not export symbol without public declaration

Signed-off-by: Olof Johansson <olof@lixom.net>

1 files changed, 15 insertions, 1 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index afb04b9b818..b38fb2b9e23 100644
--- 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)))