Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755839AbbGPSNc (ORCPT ); Thu, 16 Jul 2015 14:13:32 -0400 Received: from foss.arm.com ([217.140.101.70]:45362 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755586AbbGPSNa (ORCPT ); Thu, 16 Jul 2015 14:13:30 -0400 Date: Thu, 16 Jul 2015 19:13:27 +0100 From: Will Deacon To: Peter Zijlstra Cc: "linux-arch@vger.kernel.org" , "Waiman.Long@hp.com" , "linux-kernel@vger.kernel.org" , "paulmck@linux.vnet.ibm.com" Subject: Re: [PATCH v2 5/7] locking/qrwlock: make use of acquire/release/relaxed atomics Message-ID: <20150716181327.GT26390@arm.com> References: <1437060758-10381-1-git-send-email-will.deacon@arm.com> <1437060758-10381-6-git-send-email-will.deacon@arm.com> <20150716165903.GA19282@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150716165903.GA19282@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1865 Lines: 50 On Thu, Jul 16, 2015 at 05:59:03PM +0100, Peter Zijlstra wrote: > On Thu, Jul 16, 2015 at 04:32:36PM +0100, Will Deacon wrote: > > @@ -130,8 +130,7 @@ static inline void queued_read_unlock(struct qrwlock *lock) > > /* > > * Atomically decrement the reader count > > */ > > - smp_mb__before_atomic(); > > - atomic_sub(_QR_BIAS, &lock->cnts); > > + (void)atomic_sub_return_release(_QR_BIAS, &lock->cnts); > > } > > > > /** > > This one will actually cause different code on x86; I think its still > fine though. LOCK XADD should not be (much) slower than LOCK SUB. Yeah, I wondered whether introduced atomic_sub_release etc was worth the hassle and decided against it for now. > > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c > > index a71bb3541880..879c8fab7bea 100644 > > --- a/kernel/locking/qrwlock.c > > +++ b/kernel/locking/qrwlock.c > > @@ -36,7 +36,7 @@ rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) > > { > > while ((cnts & _QW_WMASK) == _QW_LOCKED) { > > cpu_relax_lowlatency(); > > - cnts = smp_load_acquire((u32 *)&lock->cnts); > > + cnts = atomic_read_acquire(&lock->cnts); > > } > > } > > It might make sense to add comments to the users of this function that > actually rely on the _acquire semantics, I had to double check that :-) Good point, I'll add those. > But otherwise that all looks good. Cheers. I'll send a v3 next week with your comments addressed. Pending any objection, I guess this could be merged via -tip with the exception of the ARM patch? FWIW, I plan to port arm64 once I've got my pending asm/atomic.h rework queued. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/