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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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31
kernel/events/ring_buffer.c
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@@ -87,10 +87,31 @@ again:
goto out;
/*
* Publish the known good head. Rely on the full barrier implied
* by atomic_dec_and_test() order the rb->head read and this
* write.
* Since the mmap() consumer (userspace) can run on a different CPU:
*
* kernel user
*
* READ ->data_tail READ ->data_head
* smp_mb() (A) smp_rmb() (C)
* WRITE $data READ $data
* smp_wmb() (B) smp_mb() (D)
* STORE ->data_head WRITE ->data_tail
*
* Where A pairs with D, and B pairs with C.
*
* I don't think A needs to be a full barrier because we won't in fact
* write data until we see the store from userspace. So we simply don't
* issue the data WRITE until we observe it. Be conservative for now.
*
* OTOH, D needs to be a full barrier since it separates the data READ
* from the tail WRITE.
*
* For B a WMB is sufficient since it separates two WRITEs, and for C
* an RMB is sufficient since it separates two READs.
*
* See perf_output_begin().
*/
smp_wmb();
rb->user_page->data_head = head;
/*
@@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output_handle *handle,
* Userspace could choose to issue a mb() before updating the
* tail pointer. So that all reads will be completed before the
* write is issued.
*
* See perf_output_put_handle().
*/
tail = ACCESS_ONCE(rb->user_page->data_tail);
smp_rmb();
smp_mb();
offset = head = local_read(&rb->head);
head += size;
if (unlikely(!perf_output_space(rb, tail, offset, head)))