Messages in this thread Patch in this message | | | Date | Thu, 11 Jun 2015 14:46:47 +0200 | From | Peter Zijlstra <> | Subject | [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier() |
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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.
Cc: Al Viro <viro@ZenIV.linux.org.uk> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> Suggested-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- include/linux/seqlock.h | 42 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 42 insertions(+)
--- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -233,6 +233,48 @@ static inline void raw_write_seqcount_en 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; + * + * write_seqcount_begin(seq); + * write_seqcount_end(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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