linux-kernel - Re: [PATCH -v2 3/4] locking: Introduce smp_mb__after

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Date:   Fri, 4 Aug 2017 01:43:26 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        torvalds@...ux-foundation.org, oleg@...hat.com,
        paulmck@...ux.vnet.ibm.com, benh@...nel.crashing.org,
        mpe@...erman.id.au, linux-kernel@...r.kernel.org, mingo@...nel.org,
        stern@...land.harvard.edu
Subject: Re: [PATCH -v2 3/4] locking: Introduce smp_mb__after_spinlock().

On Thu, 3 Aug 2017 16:28:20 +0100
Will Deacon <will.deacon@....com> wrote:

> On Wed, Aug 02, 2017 at 01:38:40PM +0200, Peter Zijlstra wrote:
> > Since its inception, our understanding of ACQUIRE, esp. as applied to
> > spinlocks, has changed somewhat. Also, I wonder if, with a simple
> > change, we cannot make it provide more.
> > 
> > The problem with the comment is that the STORE done by spin_lock isn't
> > itself ordered by the ACQUIRE, and therefore a later LOAD can pass over
> > it and cross with any prior STORE, rendering the default WMB
> > insufficient (pointed out by Alan).
> > 
> > Now, this is only really a problem on PowerPC and ARM64, both of
> > which already defined smp_mb__before_spinlock() as a smp_mb().
> > 
> > At the same time, we can get a much stronger construct if we place
> > that same barrier _inside_ the spin_lock(). In that case we upgrade
> > the RCpc spinlock to an RCsc.  That would make all schedule() calls
> > fully transitive against one another.
> > 
> > Cc: Alan Stern <stern@...land.harvard.edu>
> > Cc: Nicholas Piggin <npiggin@...il.com>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Michael Ellerman <mpe@...erman.id.au>
> > Cc: Oleg Nesterov <oleg@...hat.com>
> > Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/arm64/include/asm/spinlock.h   |    2 ++
> >  arch/powerpc/include/asm/spinlock.h |    3 +++
> >  include/linux/atomic.h              |    3 +++
> >  include/linux/spinlock.h            |   36 ++++++++++++++++++++++++++++++++++++
> >  kernel/sched/core.c                 |    4 ++--
> >  5 files changed, 46 insertions(+), 2 deletions(-)
> > 
> > --- a/arch/arm64/include/asm/spinlock.h
> > +++ b/arch/arm64/include/asm/spinlock.h
> > @@ -367,5 +367,7 @@ static inline int arch_read_trylock(arch
> >   * smp_mb__before_spinlock() can restore the required ordering.
> >   */
> >  #define smp_mb__before_spinlock()	smp_mb()
> > +/* See include/linux/spinlock.h */
> > +#define smp_mb__after_spinlock()	smp_mb()
> >  
> >  #endif /* __ASM_SPINLOCK_H */  
> 
> Acked-by: Will Deacon <will.deacon@....com>

Yeah this looks good to me. I don't think there would ever be a reason
to use smp_mb__before_spinlock() rather than smp_mb__after_spinlock().