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

[Paul E. McKenney]

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@xxxxxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  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@xxxxxxxxxxxxxxxxxx>

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