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seqcount: Add raw_write_seqcount_latch()

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For NMI safe access to clock monotonic we use the seqcount LSB as
index of a timekeeper array. The update sequence looks like this:

      smp_wmb();      <- prior stores to a[1]
      seq++;
      smp_wmb();      <- seq increment before update of a[0]
      update(a[0]);
      smp_wmb();      <- update of a[0]
      seq++;
      smp_wmb();      <- seq increment before update of a[1]
      update(a[1]);

To avoid open coded barriers, provide a helper function.

[ tglx: Split out of a combo patch against the first implementation of
  	the NMI safe accessor ]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
This commit is contained in:
Mathieu Desnoyers 2014-07-16 21:05:21 +00:00 committed by John Stultz
parent 0ea5a520f7
commit 9b0fd802e8
1 changed files with 11 additions and 0 deletions
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11
include/linux/seqlock.h
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@ -235,6 +235,17 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
s->sequence++;
}
/*
* raw_write_seqcount_latch - redirect readers to even/odd copy
* @s: pointer to seqcount_t
*/
static inline void raw_write_seqcount_latch(seqcount_t *s)
{
smp_wmb(); /* prior stores before incrementing "sequence" */
s->sequence++;
smp_wmb(); /* increment "sequence" before following stores */
}
/*
* Sequence counter only version assumes that callers are using their
* own mutexing.