linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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Date:	Mon, 20 Jul 2015 06:48:49 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Mon, Jul 20, 2015 at 02:39:06PM +0100, Will Deacon wrote:
> On Fri, Jul 17, 2015 at 11:14:14PM +0100, Benjamin Herrenschmidt wrote:
> > On Fri, 2015-07-17 at 10:32 +0100, Will Deacon wrote:
> > >  static inline void arch_spin_unlock(arch_spinlock_t *lock)
> > >  {
> > > -       SYNC_IO;
> > > -       __asm__ __volatile__("# arch_spin_unlock\n\t"
> > > -                               PPC_RELEASE_BARRIER: : :"memory");
> > > +       smp_mb();
> > >         lock->slock = 0;
> > >  }
> > 
> > That probably needs to be mb() in case somebody has the expectation that
> > it does a barrier vs. DMA on UP.
> 
> Hmm, but on !SMP doesn't arch_spin_unlock effectively expand to barrier()
> in the core code?

Yes, to barrier(), but that doesn't generate any code.  In contrast, the
mb() that Ben is asking for puts out a sync instruction.  Without that
sync instruction, MMIO accesses can be reordered with the spin_unlock(),
even on single-CPU systems.  So the bm() is really needed if unlock is
to order against MMIO (and thus DMA) on UP.

							Thanx, Paul

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