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perf_counter: fix race in perf_output_*

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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>
This commit is contained in:
Peter Zijlstra
2009-05-01 12:23:16 +02:00
committed by Ingo Molnar
parent 3c56999eec
commit c33a0bc4e4
2 changed files with 105 additions and 30 deletions
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5
include/linux/perf_counter.h
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@@ -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];
};