Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933045AbXEGL2t (ORCPT ); Mon, 7 May 2007 07:28:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S933120AbXEGL2s (ORCPT ); Mon, 7 May 2007 07:28:48 -0400 Received: from mail.screens.ru ([213.234.233.54]:43102 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933045AbXEGL2s (ORCPT ); Mon, 7 May 2007 07:28:48 -0400 Date: Mon, 7 May 2007 14:34:20 +0400 From: Oleg Nesterov To: Jarek Poplawski Cc: Andrew Morton , David Chinner , David Howells , Gautham Shenoy , Ingo Molnar , Srivatsa Vaddagiri , linux-kernel@vger.kernel.org Subject: Re: [PATCH] make-cancel_rearming_delayed_work-reliable-fix Message-ID: <20070507103420.GA74@tv-sign.ru> References: <20070503204226.GA212@tv-sign.ru> <20070503181513.0b0aa4fb.akpm@linux-foundation.org> <20070505213213.GA1013@tv-sign.ru> <20070507103107.GD1754@ff.dom.local> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070507103107.GD1754@ff.dom.local> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2282 Lines: 65 On 05/07, Jarek Poplawski wrote: > > There is a lot of new things in the final version of this > patch. I guess, there was no such problem in the previous > version. No, this is basically the same patch + re-check-cwq-after-lock, the latter is mostly needed to prevent racing with CPU-hotplug. > I can also see you have new doubts about usefulness, which > I cannot understand: > - even if there are some slowdowns, where does it matter? > - the "old" method uses only one method of cancelling, i.e. > del_timer, not trying to stop requeuing or to remove from > the queue; it seems to be effective only with long delayed > timers, and its real problems are probably mostly invisible. The slowdown is small, changelog mentions it just to be "fair". I am not happy with the complication this patch adds, mostly I hate this smb_wmb() in insert_work(). I have an idea how to remove it later, but this needs another patch not related to workqueue.c. > BTW, I'm still not convinced all additions are needed: > the "old" cancel_rearming_ doesn't care about checking > or waiting on anything after del_timer positive. It would be very strange to do wait_on_work() only in case when del_timer() failed. This way we still need to do cancel_work_sync() after cancel_rearming_delayed_work(), but only when del_timer() failed, ugly. Note also that wait_on_work() does not sleep if work->func() is not running. Also, consider this callback: void work_handler(struct work_struct *w) { struct delayed_work dw = container_of(...); queue_delayed_work(dw, delay); // <------------- cancel_rearming_delayed_work() cancel_delayed_work(dw); queue_delayed_work(dw, another_delay); } Yes, this is strange and ugly. But correct! The current version (before this patch) can't cancel this delayed_work. The new implementation works correctly. So I think it is far better to do wait_on_work() unconditionally. > PS: I'll try to check this all in the evening and will > write tomorrow, if found something interesting. Yes, please! Oleg. - 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/