From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: [PATCH 7/9] aio: add delayed cancel support Date: Thu, 22 Mar 2018 16:33:56 +0000 Message-ID: <20180322163356.GG30522@ZenIV.linux.org.uk> References: <20180321073232.13366-1-hch@lst.de> <20180321073232.13366-8-hch@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20180321073232.13366-8-hch@lst.de> Sender: owner-linux-aio@kvack.org To: Christoph Hellwig Cc: Avi Kivity , linux-aio@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-api@vger.kernel.org On Wed, Mar 21, 2018 at 08:32:30AM +0100, Christoph Hellwig wrote: > The upcoming aio poll support would like to be able to complete the > iocb inline from the cancellation context, but that would cause > a lock order reversal. Add support for optionally moving the cancelation > outside the context lock to avoid this reversal. Ouch... Seeing that you've just taken out cmpxchg loop out of kiocb_cancel() with "serialized on ->ctx_lock" for explanation of safety... Let me check the aio_poll side of it; this commit might be better off in the poll series, *if* it is actually correct. What's to prevent double completions there? Suppose we have iocb sitting on the wait queue; cancellation callback set, so's "delayed cancel" flag. Now, somebody tries to cancel the fucker on CPU1. With ctx->lock held the sucker is found on the list and, just as we mark it "cancelled", driver sends a wakeup, executing (on CPU2) aio_poll_wake(), calling aio_complete_poll() (without ctx->lock, so no exclusion with io_cancel(2) on CPU1), which checks AIO_IOCB_CANCELLED and does not notice the flag being set on CPU1, then proceeds to __aio_complete_poll() and fput() in there. In the meanwhile, CPU1 has taken the sucker off the list, dropped the lock and called kiocb_cancel() on it. Now we get aio_poll_cancel() and __aio_complete_poll() on CPU1, with *another* fput(). What am I missing here that would prevent such a race? -- To unsubscribe, send a message with 'unsubscribe linux-aio' in the body to majordomo@kvack.org. For more info on Linux AIO, see: http://www.kvack.org/aio/ Don't email: aart@kvack.org