Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755049AbaAUPpy (ORCPT ); Tue, 21 Jan 2014 10:45:54 -0500 Received: from g4t0015.houston.hp.com ([15.201.24.18]:20318 "EHLO g4t0015.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754760AbaAUPpu (ORCPT ); Tue, 21 Jan 2014 10:45:50 -0500 Message-ID: <52DE9626.3030806@hp.com> Date: Tue, 21 Jan 2014 10:45:42 -0500 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Peter Zijlstra 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 , "" , Scott J Norton Subject: Re: [PATCH v9 4/5] qrwlock: Use smp_store_release() in write_unlock() References: <1389761047-47566-1-git-send-email-Waiman.Long@hp.com> <1389761047-47566-5-git-send-email-Waiman.Long@hp.com> <20140120151855.GH30183@twins.programming.kicks-ass.net> In-Reply-To: <20140120151855.GH30183@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/20/2014 10:18 AM, Peter Zijlstra wrote: > 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: That is not true. A reader may transiently set the reader count to a non-zero value in the fast path. Also, a reader in interrupt context will force a non-zero reader count to take the read lock as soon as the writer is done. > > 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? > The current code is optimized for the reader-heavy case. So I used xadd for incrementing reader count to reduce the chance of retry due to concurrent reader count updates. The downside is the need to back out if a writer is here. I can change the logic to use only cmpxchg for readers, but I don't see a compelling reason to do so. -Longman -- 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/