Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933786AbbBCL1j (ORCPT ); Tue, 3 Feb 2015 06:27:39 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:53459 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754958AbbBCL1i (ORCPT ); Tue, 3 Feb 2015 06:27:38 -0500 Date: Tue, 3 Feb 2015 12:27:33 +0100 From: Peter Zijlstra To: Linus Torvalds Cc: Benjamin LaHaise , linux-aio@kvack.org, Linux Kernel Subject: Re: [GIT PULL] aio: fix sleeping while TASK_INTERRUPTIBLE Message-ID: <20150203112733.GM26304@twins.programming.kicks-ass.net> References: <20150201144058.GM2974@kvack.org> <20150201221458.GN2974@kvack.org> <20150202001628.GO2974@kvack.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3196 Lines: 72 On Sun, Feb 01, 2015 at 05:18:17PM -0800, Linus Torvalds wrote: > Ahh. That would be a bug, yes, but it wouldn't be one in the aio code. > > If somebody just does a "schedule()" and thinks that their own private > events are the only thing that can wake it up, and doesn't use one of > the millions of "wait_event_xyz()" variations to actually wait for the > real completion, that is just buggy. Always. Always has been. > > So I wouldn't worry too much about it. It has never been correct to do > that, and it's not one of the standard patterns for sleeping anyway. > Which is not to say that it might not exist in some dank corner of the > kernel, of course, but you shouldn't write code trying to make buggy > code like that happy. If we ever find code like that, let's just fix > it where the bug exists, not try to write odd code in places where it > isn't. > > And I'd actually be a bit surprised to see that kind of really broken > code. You really almost have to work at it. All our normal "sleep > until X" patterns are much more obvious, and it's just _simpler_ to do > the right thing with "wait_event()" than to mis-code an explicit "set > task state and then just schedule without actually checking the thing > you are waiting for". block/bsg.c- prepare_to_wait(&bd->wq_done, &wait, TASK_UNINTERRUPTIBLE); block/bsg.c- spin_unlock_irq(&bd->lock); block/bsg.c: io_schedule(); block/bsg.c- finish_wait(&bd->wq_done, &wait); Which is double buggy because: 1) it doesn't loop 2) it sets TASK_UNINTERRUPTIBLE _after_ testing for the sleep event. drivers/iommu/amd_iommu_v2.c-static void put_device_state_wait(struct device_state *dev_state) drivers/iommu/amd_iommu_v2.c-{ drivers/iommu/amd_iommu_v2.c- DEFINE_WAIT(wait); drivers/iommu/amd_iommu_v2.c- drivers/iommu/amd_iommu_v2.c- prepare_to_wait(&dev_state->wq, &wait, TASK_UNINTERRUPTIBLE); drivers/iommu/amd_iommu_v2.c- if (!atomic_dec_and_test(&dev_state->count)) drivers/iommu/amd_iommu_v2.c: schedule(); drivers/iommu/amd_iommu_v2.c- finish_wait(&dev_state->wq, &wait); drivers/iommu/amd_iommu_v2.c- drivers/iommu/amd_iommu_v2.c- free_device_state(dev_state); drivers/iommu/amd_iommu_v2.c-} No loop... drivers/md/dm-bufio.c-static void __wait_for_free_buffer(struct dm_bufio_client *c) drivers/md/dm-bufio.c-{ drivers/md/dm-bufio.c- DECLARE_WAITQUEUE(wait, current); drivers/md/dm-bufio.c- drivers/md/dm-bufio.c- add_wait_queue(&c->free_buffer_wait, &wait); drivers/md/dm-bufio.c- set_task_state(current, TASK_UNINTERRUPTIBLE); drivers/md/dm-bufio.c- dm_bufio_unlock(c); drivers/md/dm-bufio.c- drivers/md/dm-bufio.c: io_schedule(); drivers/md/dm-bufio.c- drivers/md/dm-bufio.c- remove_wait_queue(&c->free_buffer_wait, &wait); drivers/md/dm-bufio.c- drivers/md/dm-bufio.c- dm_bufio_lock(c); drivers/md/dm-bufio.c-} No loop.. And I'm sure there's a _waaay_ more. And the above examples are all in relatively well maintained code afaik. -- 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/