Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753519AbaATPTj (ORCPT ); Mon, 20 Jan 2014 10:19:39 -0500 Received: from merlin.infradead.org ([205.233.59.134]:59350 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751868AbaATPTf (ORCPT ); Mon, 20 Jan 2014 10:19:35 -0500 Date: Mon, 20 Jan 2014 16:18:55 +0100 From: Peter Zijlstra To: Waiman Long Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Arnd Bergmann , linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, Steven Rostedt , Andrew Morton , Michel Lespinasse , Andi Kleen , Rik van Riel , "Paul E. McKenney" , Linus Torvalds , Raghavendra K T , George Spelvin , Tim Chen , "Aswin Chandramouleeswaran\"" , Scott J Norton Subject: Re: [PATCH v9 4/5] qrwlock: Use smp_store_release() in write_unlock() Message-ID: <20140120151855.GH30183@twins.programming.kicks-ass.net> References: <1389761047-47566-1-git-send-email-Waiman.Long@hp.com> <1389761047-47566-5-git-send-email-Waiman.Long@hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1389761047-47566-5-git-send-email-Waiman.Long@hp.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 On Tue, Jan 14, 2014 at 11:44:06PM -0500, Waiman Long wrote: > This patch modifies the queue_write_unlock() function to use the new > smp_store_release() function (currently in tip). It also removes the > temporary implementation of smp_load_acquire() and smp_store_release() > function in qrwlock.c. > > This patch will use atomic subtraction instead if the writer field is > not atomic. > > Signed-off-by: Waiman Long > --- > include/asm-generic/qrwlock.h | 10 ++++++---- > kernel/locking/qrwlock.c | 34 ---------------------------------- > 2 files changed, 6 insertions(+), 38 deletions(-) > > diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h > index 5abb6ca..68f488b 100644 > --- a/include/asm-generic/qrwlock.h > +++ b/include/asm-generic/qrwlock.h > @@ -181,11 +181,13 @@ static inline void queue_read_unlock(struct qrwlock *lock) > static inline void queue_write_unlock(struct qrwlock *lock) > { > /* > - * Make sure that none of the critical section will be leaked out. > + * If the writer field is atomic, it can be cleared directly. > + * Otherwise, an atomic subtraction will be used to clear it. > */ > - smp_mb__before_clear_bit(); > - ACCESS_ONCE(lock->cnts.writer) = 0; > - smp_mb__after_clear_bit(); > + if (__native_word(lock->cnts.writer)) > + smp_store_release(&lock->cnts.writer, 0); > + else > + atomic_sub(_QW_LOCKED, &lock->cnts.rwa); > } If we're a writer, read-count must be zero. The only way for that not to be zero is a concurrent read-(try)lock. If you move all the read-(try)locks over to cmpxchg() you can avoid this afaict: static inline void queue_read_trylock(struct qrwlock *lock) { union qrwcnts cnts cnts = ACCESS_ONCE(lock->cnts); if (!cnts.writer) { if (cmpxchg(&lock->cnts.rwc, cnts.rwc, cnts.rwc + _QR_BIAS) == cnts.rwc) return 1; } return 0; } static inline void queue_read_lock(struct qrwlock *lock) { if (!queue_read_trylock(lock)) queue_read_lock_slowpath(); // XXX do not assume extra _QR_BIAS } At which point you have the guarantee that read-count == 0, and you can write: static inline void queue_write_unlock(struct qrwlock *lock) { smp_store_release(&lock->cnts.rwc, 0); } No? -- 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/