From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with SMTP id 48B316B0044 for ; Fri, 23 Jan 2009 07:36:36 -0500 (EST) Received: by mu-out-0910.google.com with SMTP id i2so3143712mue.6 for ; Fri, 23 Jan 2009 04:36:34 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20090123110500.GA12684@redhat.com> References: <20090117215110.GA3300@redhat.com> <20090118023211.GA14539@redhat.com> <20090120203131.GA20985@cmpxchg.org> <20090121143602.GA16584@redhat.com> <20090121213813.GB23270@cmpxchg.org> <20090122202550.GA5726@redhat.com> <20090123004702.GA18362@redhat.com> <20090123110500.GA12684@redhat.com> Date: Fri, 23 Jan 2009 13:36:33 +0100 Message-ID: Subject: Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock From: Dmitry Adamushko Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Oleg Nesterov Cc: Johannes Weiner , Chris Mason , Peter Zijlstra , Matthew Wilcox , Chuck Lever , Nick Piggin , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Ingo Molnar List-ID: 2009/1/23 Oleg Nesterov : > On 01/23, Dmitry Adamushko wrote: >> >> 2009/1/23 Oleg Nesterov : >> > On 01/23, Dmitry Adamushko wrote: >> >> >> >> In short, wq->lock is a sync. mechanism in this case. The scheme is as follows: >> >> >> >> our side: >> >> >> >> [ finish_wait() ] >> >> >> >> lock(wq->lock); >> > >> > But we can skip lock(wq->lock), afaics. >> > >> > Without rmb(), test_bit() can be re-ordered with list_empty_careful() >> > in finish_wait() and even with __set_task_state(TASK_RUNNING). >> >> But taking into account the constraints of this special case, namely >> (1), we can't skip lock(wq->lock). >> >> (1) "the next contender is us" >> >> In this particular situation, we are only interested in the case when >> we were woken up by __wake_up_bit(). > > Yes, > >> that means we are _on_ the 'wq' list when we do finish_wait() -> we do >> take the 'wq->lock'. > > Hmm. No? > > We are doing exclusive wait, and we use autoremove_wake_function(). > If we were woken, we are removed from ->task_list. Argh, right, somehow I've made wrong assumptions on the wake-up part :-/ > > Oleg. > -- Best regards, Dmitry Adamushko -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org