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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>