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perf: Fix races in computing the header sizes 

There are two races with the current code:

 - Another event can join the group and compute a larger header_size
   concurrently, if the smaller store wins we'll have an incorrect
   header_size set.

 - We compute the header_size after the event becomes active,
   therefore its possible to use the size before its computed.

Remedy the first by moving the computation inside the ctx::mutex lock,
and the second by placing it _before_ perf_install_in_context().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2015-09-09 20:48:22 +02:00 committed by Ingo Molnar
parent a723968c0e
commit f73e22ab45
1 changed files with 9 additions and 6 deletions
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15
kernel/events/core.c
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@ -8399,6 +8399,15 @@ SYSCALL_DEFINE5(perf_event_open,
put_ctx(gctx);
}
/*
* Precalculate sample_data sizes; do while holding ctx::mutex such
* that we're serialized against further additions and before
* perf_install_in_context() which is the point the event is active and
* can use these values.
*/
perf_event__header_size(event);
perf_event__id_header_size(event);
perf_install_in_context(ctx, event, event->cpu);
perf_unpin_context(ctx);
@ -8414,12 +8423,6 @@ SYSCALL_DEFINE5(perf_event_open,
list_add_tail(&event->owner_entry, &current->perf_event_list);
mutex_unlock(&current->perf_event_mutex);
/*
* Precalculate sample_data sizes
*/
perf_event__header_size(event);
perf_event__id_header_size(event);
/*
* Drop the reference on the group_event after placing the
* new event on the sibling_list. This ensures destruction