Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1032407AbWLGQzL (ORCPT ); Thu, 7 Dec 2006 11:55:11 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1032411AbWLGQzL (ORCPT ); Thu, 7 Dec 2006 11:55:11 -0500 Received: from smtp.osdl.org ([65.172.181.25]:43555 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1032407AbWLGQzJ (ORCPT ); Thu, 7 Dec 2006 11:55:09 -0500 Date: Thu, 7 Dec 2006 08:54:09 -0800 From: Andrew Morton To: David Howells Cc: torvalds@osdl.org, davem@davemloft.com, wli@holomorphy.com, matthew@wil.cx, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: [PATCH 3/3] WorkStruct: Use direct assignment rather than cmpxchg() Message-Id: <20061207085409.228016a2.akpm@osdl.org> In-Reply-To: <20061207153143.28408.7274.stgit@warthog.cambridge.redhat.com> References: <20061207153138.28408.94099.stgit@warthog.cambridge.redhat.com> <20061207153143.28408.7274.stgit@warthog.cambridge.redhat.com> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2821 Lines: 71 On Thu, 07 Dec 2006 15:31:43 +0000 David Howells wrote: > Use direct assignment rather than cmpxchg() as the latter is unavailable and > unimplementable on some platforms and is actually unnecessary. > > The use of cmpxchg() was to guard against two possibilities, neither of which > can actually occur: > > (1) The pending flag may have been unset or may be cleared. However, given > where it's called, the pending flag is _always_ set. I don't think it > can be unset whilst we're in set_wq_data(). > > Once the work is enqueued to be actually run, the only way off the queue > is for it to be actually run. > > If it's a delayed work item, then the bit can't be cleared by the timer > because we haven't started the timer yet. Also, the pending bit can't be > cleared by cancelling the delayed work _until_ the work item has had its > timer started. > > (2) The workqueue pointer might change. This can only happen in two cases: > > (a) The work item has just been queued to actually run, and so we're > protected by the appropriate workqueue spinlock. > > (b) A delayed work item is being queued, and so the timer hasn't been > started yet, and so no one else knows about the work item or can > access it (the pending bit protects us). > > Besides, set_wq_data() _sets_ the workqueue pointer unconditionally, so > it can be assigned instead. > > So, replacing the set_wq_data() with a straight assignment would be okay in > most cases. The problem is where we end up tangling with test_and_set_bit() > emulated using spinlocks, and even then it's not a problem _provided_ > test_and_set_bit() doesn't attempt to modify the word if the bit was set. > > If that's a problem, then a bitops-proofed assignment will be required - > equivalent to atomic_set() vs other atomic_xxx() ops. > I don't understand, as usual. afacit in all (but one) cases we do if (!test_and_set_bit(WORK_STRUCT_PENDING, &work->management)) { ... set_wq_data(work, wq); ... } cancel_delayed_work() looks OK too. The possible exception is schedule_on_each_cpu() which is being lazy, but looks fixable. So... afaict in all the places where there can be a concurrent set_wq_data() and test_and_set_bit(), WORK_STRUCT_PENDING is reliably set, and we can assume (and ensure) that a failing test_and_set_bit() will not write to the affected word at all. What am I missing? - 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/