From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id 7D6066B0044 for ; Fri, 23 Jan 2009 06:07:32 -0500 (EST) Date: Fri, 23 Jan 2009 12:05:00 +0100 From: Oleg Nesterov Subject: Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock Message-ID: <20090123110500.GA12684@redhat.com> References: <20090117215110.GA3300@redhat.com> <20090118013802.GA12214@cmpxchg.org> <20090118023211.GA14539@redhat.com> <20090120203131.GA20985@cmpxchg.org> <20090121143602.GA16584@redhat.com> <20090121213813.GB23270@cmpxchg.org> <20090122202550.GA5726@redhat.com> <20090123004702.GA18362@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: Dmitry Adamushko 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: 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. > Moreover, imagine the following case (roughly similar to finish_wait()): > > if (LOAD(a) == 1) { > // do something here > mb(); > } > > LOAD(b); > > Can LOAD(b) be reordered with LOAD(a)? Well, I think yes it can. But I'd suggest you to ask somebody else ;) So, without rmb() I think it is theoretically possible that we read test_bit() before we get list_empty_careful() == T. Oleg. -- 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