Messages in this thread Patch in this message | | | Date | Thu, 18 Jun 2015 15:19:27 -0700 | From | tip-bot for Peter Zijlstra <> | Subject | [tip:timers/core] seqcount: Introduce raw_write_seqcount_barrier( ) |
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Commit-ID: c4bfa3f5f906aee2e084c5b1fb15caf876338ef8 Gitweb: http://git.kernel.org/tip/c4bfa3f5f906aee2e084c5b1fb15caf876338ef8 Author: Peter Zijlstra <peterz@infradead.org> AuthorDate: Wed, 17 Jun 2015 14:29:24 +0200 Committer: Thomas Gleixner <tglx@linutronix.de> CommitDate: Fri, 19 Jun 2015 00:09:56 +0200
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: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter 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.net Signed-off-by: Thomas Gleixner <tglx@linutronix.de> --- include/linux/seqlock.h | 41 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 41 insertions(+)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index c07e3a5..486e685 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -233,6 +233,47 @@ static inline void raw_write_seqcount_end(seqcount_t *s) 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 * @s: pointer to seqcount_t
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