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Re: [v3,11/41] mips: reuse asm-generic/barrier.h

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Hi Paul,

On Mon, Jan 18, 2016 at 07:46:29AM -0800, Paul E. McKenney wrote:
> On Mon, Jan 18, 2016 at 04:19:29PM +0800, Herbert Xu wrote:
> > Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > >
> > > You could use SYNC_ACQUIRE() to implement read_barrier_depends() and
> > > smp_read_barrier_depends(), but SYNC_RMB probably does not suffice.
> > > The reason for this is that smp_read_barrier_depends() must order the
> > > pointer load against any subsequent read or write through a dereference
> > > of that pointer.  For example:
> > > 
> > >        p = READ_ONCE(gp);
> > >        smp_rmb();
> > >        r1 = p->a; /* ordered by smp_rmb(). */
> > >        p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */
> > >        r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */
> > > 
> > > In contrast:
> > > 
> > >        p = READ_ONCE(gp);
> > >        smp_read_barrier_depends();
> > >        r1 = p->a; /* ordered by smp_read_barrier_depends(). */
> > >        p->b = 42; /* ordered by smp_read_barrier_depends(). */
> > >        r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */
> > > 
> > > Again, if your hardware maintains local ordering for address
> > > and data dependencies, you can have read_barrier_depends() and
> > > smp_read_barrier_depends() be no-ops like they are for most
> > > architectures.
> > > 
> > > Does that help?
> > 
> > This is crazy! smp_rmb started out being strictly stronger than
> > smp_read_barrier_depends, when did this stop being the case?
> 
> Hello, Herbert!
> 
> It is true that most Linux kernel code relies only on the read-read
> properties of dependencies, but the read-write properties are useful.
> Admittedly relatively rarely, but useful.
> 
> The better comparison for smp_read_barrier_depends(), especially in
> its rcu_dereference*() form, is smp_load_acquire().
> 

Confused..

I recall that last time you and Linus came into a conclusion that even
on Alpha, a barrier for read->write with data dependency is unnecessary:

http://article.gmane.org/gmane.linux.kernel/2077661

And in an earlier mail of that thread, Linus made his point that
smp_read_barrier_depends() should only be used to order read->read.

So right now, are we going to extend the semantics of
smp_read_barrier_depends()? Can we just make smp_read_barrier_depends()
still only work for read->read, and assume all the architectures won't
reorder read->write with data dependency, so that the code above having
a smp_rmb() also works?

Regards,
Boqun

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Re: [v3,11/41] mips: reuse asm-generic/barrier.h Herbert Xu <herbert@gondor.apana.org.au> - 2016-01-18 09:30 +0100
  Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-18 16:50 +0100
    Re: [v3,11/41] mips: reuse asm-generic/barrier.h Boqun Feng <boqun.feng@gmail.com> - 2016-01-26 18:00 +0100
      Re: [v3,11/41] mips: reuse asm-generic/barrier.h Peter Zijlstra <peterz@infradead.org> - 2016-01-26 18:30 +0100
        Re: [v3,11/41] mips: reuse asm-generic/barrier.h Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-26 20:50 +0100
          Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-26 23:10 +0100
            Re: [v3,11/41] mips: reuse asm-generic/barrier.h Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-26 23:20 +0100
              Re: [v3,11/41] mips: reuse asm-generic/barrier.h Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-26 23:40 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-27 00:30 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h Linus Torvalds <torvalds@linux-foundation.org> - 2016-01-27 00:50 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-27 02:00 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h Boqun Feng <boqun.feng@gmail.com> - 2016-01-27 03:10 +0100
                Re: [v3,11/41] mips: reuse asm-generic/barrier.h Peter Zijlstra <peterz@infradead.org> - 2016-01-27 09:00 +0100
      Re: [v3,11/41] mips: reuse asm-generic/barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-01-26 23:10 +0100

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