Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier()

Paul E. McKenney
2015-06-11T15:33:51Z

On Thu, Jun 11, 2015 at 02:46:47PM +0200, Peter Zijlstra wrote:
> 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;
> + * }

So when using this, write() would instead look like this?

void write(void)
{
Y = true;
raw_write_seqcount_barrier(seq);
X = false;
}

I suggest calling this out explicitly. Agreed, it should be obvious,
but some poor sot is going to be reading this at 3AM local time after
a couple days of no sleep, in which case obvious might not be so obvious.

I also would suggest READ_ONCE() and WRITE_ONCE() to keep the compiler
trickiness down to a dull roar. Understood, it is hard to make anything
bad happen in this case, but small changes could result in badness.

> + */
> +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

Looks good otherwise.

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>