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

[Paul E. McKenney]

On Thu, Jun 11, 2015 at 08:33:41AM -0700, Paul E. McKenney wrote:
> 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>

Color me slow and stupid.  Maybe due to reviewing a patch too early in
the morning, who knows?

There is nothing above that prevents the compiler and the CPU from
reordering the assignments to X and Y with the increment of s->sequence++.
One fix would be as follows:

	static inline void raw_write_seqcount_barrier(seqcount_t *s)
	{
		smp_wmb();
		s->sequence++;
		smp_wmb();
		s->sequence++;
		smp_wmb();
	}

Of course, this assumes that the accesses surrounding the call to
raw_write_seqcount_barrier() are writes.  If they can be a reads,
the two added smp_wmb() calls need to be full barriers.

							Thanx, Paul

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