linux-kernel - Re: [PATCH v5 3/3] locking/qrwlock: Don't contend with readers when setting _QW

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Date:	Tue, 23 Jun 2015 09:37:56 +0100
From:	Will Deacon <will.deacon@....com>
To:	Waiman Long <waiman.long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v5 3/3] locking/qrwlock: Don't contend with readers when
 setting _QW_WAITING

On Tue, Jun 23, 2015 at 03:57:48AM +0100, Waiman Long wrote:
> On 06/22/2015 12:21 PM, Will Deacon wrote:
> > On Fri, Jun 19, 2015 at 04:50:02PM +0100, Waiman Long wrote:
> >> The current cmpxchg() loop in setting the _QW_WAITING flag for writers
> >> in queue_write_lock_slowpath() will contend with incoming readers
> >> causing possibly extra cmpxchg() operations that are wasteful. This
> >> patch changes the code to do a byte cmpxchg() to eliminate contention
> >> with new readers.
> > [...]
> >
> >> diff --git a/arch/x86/include/asm/qrwlock.h b/arch/x86/include/asm/qrwlock.h
> >> index a8810bf..5678b0a 100644
> >> --- a/arch/x86/include/asm/qrwlock.h
> >> +++ b/arch/x86/include/asm/qrwlock.h
> >> @@ -7,8 +7,7 @@
> >>   #define queued_write_unlock queued_write_unlock
> >>   static inline void queued_write_unlock(struct qrwlock *lock)
> >>   {
> >> -        barrier();
> >> -        ACCESS_ONCE(*(u8 *)&lock->cnts) = 0;
> >> +	smp_store_release(&lock->wmode, 0);
> >>   }
> >>   #endif
> > I reckon you could actually use this in the asm-generic header and remove
> > the x86 arch version altogether. Most architectures support single-copy
> > atomic byte access and those that don't (alpha?) can just not use qrwlock
> > (or override write_unlock with atomic_sub).
> >
> > I already have a patch making this change, so I'm happy either way.
> 
> Yes, I am aware of that. If you have a patch to make that change, I am 
> fine with that too.

Tell you what; I'll rebase my patches on top of yours and post them after
the merge window.

Will
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