Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754087AbbGPQ7L (ORCPT ); Thu, 16 Jul 2015 12:59:11 -0400 Received: from casper.infradead.org ([85.118.1.10]:44102 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753375AbbGPQ7K (ORCPT ); Thu, 16 Jul 2015 12:59:10 -0400 Date: Thu, 16 Jul 2015 18:59:03 +0200 From: Peter Zijlstra To: Will Deacon 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: <20150716165903.GA19282@twins.programming.kicks-ass.net> References: <1437060758-10381-1-git-send-email-will.deacon@arm.com> <1437060758-10381-6-git-send-email-will.deacon@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1437060758-10381-6-git-send-email-will.deacon@arm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1352 Lines: 38 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. > 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 :-) But otherwise that all looks good. -- 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/