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perf: Fix perf ring buffer memory ordering

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The PPC64 people noticed a missing memory barrier and crufty old
comments in the perf ring buffer code. So update all the comments and
add the missing barrier.

When the architecture implements local_t using atomic_long_t there
will be double barriers issued; but short of introducing more
conditional barrier primitives this is the best we can do.

Reported-by: Victor Kaplansky <victork@il.ibm.com>
Tested-by: Victor Kaplansky <victork@il.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: michael@ellerman.id.au
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Michael Neuling <mikey@neuling.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: anton@samba.org
Cc: benh@kernel.crashing.org
Link: http://lkml.kernel.org/r/20131025173749.GG19466@laptop.lan
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
2013-10-28 13:55:29 +01:00
committed by Ingo Molnar
parent cd65718712
commit bf378d341e
2 changed files with 34 additions and 9 deletions
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12
include/uapi/linux/perf_event.h
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@@ -456,13 +456,15 @@ struct perf_event_mmap_page {
/*
* Control data for the mmap() data buffer.
*
* User-space reading the @data_head value should issue an rmb(), on
* SMP capable platforms, after reading this value -- see
* perf_event_wakeup().
* User-space reading the @data_head value should issue an smp_rmb(),
* after reading this value.
*
* When the mapping is PROT_WRITE the @data_tail value should be
* written by userspace to reflect the last read data. In this case
* the kernel will not over-write unread data.
* written by userspace to reflect the last read data, after issueing
* an smp_mb() to separate the data read from the ->data_tail store.
* In this case the kernel will not over-write unread data.
*
* See perf_output_put_handle() for the data ordering.
*/
__u64 data_head; /* head in the data section */
__u64 data_tail; /* user-space written tail */