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Date: Mon, 22 Jun 2015 22:57:48 -0400 From: Waiman Long <waiman.long@...com> To: Will Deacon <will.deacon@....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 06/22/2015 12:21 PM, Will Deacon wrote: > Hi Waiman, > > 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. Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/
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