Commit c4bfa3f5 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner

seqcount: Introduce raw_write_seqcount_barrier()

Introduce raw_write_seqcount_barrier(), a new construct that can be
used to provide write barrier semantics in seqcount read loops instead
of the usual consistency guarantee.

raw_write_seqcount_barier() is equivalent to:

	raw_write_seqcount_begin();
	raw_write_seqcount_end();

But avoids issueing two back-to-back smp_wmb() instructions.

This construct works because the read side will 'stall' when observing
odd values. This means that -- referring to the example in the comment
below -- even though there is no (matching) read barrier between the
loads of X and Y, we cannot observe !x && !y, because:

 - if we observe Y == false we must observe the first sequence
   increment, which makes us loop, until

 - we observe !(seq & 1) -- the second sequence increment -- at which
   time we must also observe T == true.
Suggested-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: umgwanakikbuti@gmail.com
Cc: ktkhai@parallels.com
Cc: rostedt@goodmis.org
Cc: juri.lelli@gmail.com
Cc: pang.xunlei@linaro.org
Cc: oleg@redhat.com
Cc: wanpeng.li@linux.intel.com
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20150617122924.GP3644@twins.programming.kicks-ass.netSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent a7c6f571
...@@ -233,6 +233,47 @@ static inline void raw_write_seqcount_end(seqcount_t *s) ...@@ -233,6 +233,47 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
s->sequence++; s->sequence++;
} }
/**
* raw_write_seqcount_barrier - do a seq write barrier
* @s: pointer to seqcount_t
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
* collapse the two back-to-back wmb()s.
*
* seqcount_t seq;
* bool X = true, Y = false;
*
* void read(void)
* {
* bool x, y;
*
* do {
* int s = read_seqcount_begin(&seq);
*
* x = X; y = Y;
*
* } while (read_seqcount_retry(&seq, s));
*
* BUG_ON(!x && !y);
* }
*
* void write(void)
* {
* Y = true;
*
* raw_write_seqcount_barrier(seq);
*
* X = false;
* }
*/
static inline void raw_write_seqcount_barrier(seqcount_t *s)
{
s->sequence++;
smp_wmb();
s->sequence++;
}
/* /*
* raw_write_seqcount_latch - redirect readers to even/odd copy * raw_write_seqcount_latch - redirect readers to even/odd copy
* @s: pointer to seqcount_t * @s: pointer to seqcount_t
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment