From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932166AbcEKJbd (ORCPT ); Wed, 11 May 2016 05:31:33 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:33265 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750954AbcEKJbb (ORCPT ); Wed, 11 May 2016 05:31:31 -0400 Date: Wed, 11 May 2016 11:31:27 +0200 From: Michal Hocko To: Peter Zijlstra Cc: Tetsuo Handa , LKML , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , "David S. Miller" , Tony Luck , Andrew Morton , Chris Zankel , Max Filippov , Davidlohr Bueso , Waiman Long Subject: Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Message-ID: <20160511093127.GI16677@dhcp22.suse.cz> References: <1459508695-14915-1-git-send-email-mhocko@kernel.org> <1459508695-14915-4-git-send-email-mhocko@kernel.org> <8bd03bdc-0373-a3bb-da12-045322efb797@I-love.SAKURA.ne.jp> <20160510115320.GJ23576@dhcp22.suse.cz> <20160510123806.GB3193@twins.programming.kicks-ass.net> <20160511072357.GC16677@dhcp22.suse.cz> <20160511082853.GF16677@dhcp22.suse.cz> <20160511084401.GH3193@twins.programming.kicks-ass.net> <20160511090442.GH16677@dhcp22.suse.cz> <20160511091733.GC3192@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160511091733.GC3192@twins.programming.kicks-ass.net> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 11-05-16 11:17:33, Peter Zijlstra wrote: > On Wed, May 11, 2016 at 11:04:42AM +0200, Michal Hocko wrote: > > On Wed 11-05-16 10:44:01, Peter Zijlstra wrote: > > [...] > > > @@ -504,6 +502,18 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) > > > raw_spin_unlock_irq(&sem->wait_lock); > > > > > > return ret; > > > + > > > +out_nolock: > > > + __set_current_state(TASK_RUNNING); > > > + raw_spin_lock_irq(&sem->wait_lock); > > > + list_del(&waiter.list); > > > + if (list_empty(&sem->wait_list)) > > > + rwsem_atomic_update(-RWSEM_WAITING_BIAS, sem); > > > + else > > > + __rwsem_do_wake(sem, RWSEM_WAKE_READERS); > > > + raw_spin_unlock_irq(&sem->wait_lock); > > > + > > > + return ERR_PTR(-EINTR); > > > } > > > > Looks much better but don't we have to check the count for potentially > > pending writers? > > Ah, so I was thinking that if we get here, there must still be an owner, > otherwise we'd have acquired the lock. And if there is an owner, we > cannot go wake writers. Hence the WAKE_READERS thing. I was worried about the case where the owner is writer and we would wake readers but I have missed that this wouldn't happen because of if (wake_type != RWSEM_WAKE_READ_OWNED) { adjustment = RWSEM_ACTIVE_READ_BIAS; try_reader_grant: oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { /* A writer stole the lock. Undo our reader grant. */ if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) goto out; /* Last active locker left. Retry waking readers. */ goto try_reader_grant; } } > Then again, WAKE_ANY would not harm, in that if we do wake a pending > writer it will not proceed if it cannot and it'll go back to sleep > again. true > So yeah, maybe WAKE_ANY is the prudent thing to do. I guess so. Care to cook up a full patch? Thanks! -- Michal Hocko SUSE Labs