perf_counter: fix race in perf_output_*

When two (or more) contexts output to the same buffer, it is possible
to observe half written output.

Suppose we have CPU0 doing perf_counter_mmap(), CPU1 doing
perf_counter_overflow(). If CPU1 does a wakeup and exposes head to
user-space, then CPU2 can observe the data CPU0 is still writing.

[ Impact: fix occasionally corrupted profiling records ]

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090501102533.007821627@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 4 insertions, 1 deletions

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 41aed4270057..f776851f8c4b 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -358,10 +358,13 @@ struct perf_mmap_data {
         struct rcu_head                 rcu_head;
         int                             nr_pages;       /* nr of data pages  */
 
-        atomic_t                        wakeup;         /* POLL_ for wakeups */
+        atomic_t                        poll;           /* POLL_ for wakeups */
         atomic_t                        head;           /* write position    */
         atomic_t                        events;         /* event limit       */
 
+        atomic_t                        wakeup_head;    /* completed head    */
+        atomic_t                        lock;           /* concurrent writes */
+
         struct perf_counter_mmap_page   *user_page;
         void                            *data_pages[0];
 };