From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 554AAC18E5B for ; Mon, 16 Mar 2020 12:14:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2EC2D2051A for ; Mon, 16 Mar 2020 12:14:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730909AbgCPMOT (ORCPT ); Mon, 16 Mar 2020 08:14:19 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:57627 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730889AbgCPMOS (ORCPT ); Mon, 16 Mar 2020 08:14:18 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04452;MF=xiaoguang.wang@linux.alibaba.com;NM=1;PH=DS;RN=3;SR=0;TI=SMTPD_---0Tsn-vqS_1584360845; Received: from 30.5.112.45(mailfrom:xiaoguang.wang@linux.alibaba.com fp:SMTPD_---0Tsn-vqS_1584360845) by smtp.aliyun-inc.com(127.0.0.1); Mon, 16 Mar 2020 20:14:05 +0800 To: io-uring@vger.kernel.org Cc: "axboe@kernel.dk" , joseph qi From: Xiaoguang Wang Subject: bug report about patch "io_uring: avoid ring quiesce for fixed file set unregister and update" Message-ID: <2b1834e3-1870-66a4-bbf4-70ab8a5109a6@linux.alibaba.com> Date: Mon, 16 Mar 2020 20:14:05 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 Content-Type: text/plain; charset=gbk; format=flowed Content-Transfer-Encoding: 7bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org hi, While diving into iouring file register/unregister/update codes, seems that there is one bug in __io_sqe_files_update(): if (ref_switch) percpu_ref_switch_to_atomic(&data->refs, io_atomic_switch); The initial fixed_file_data's refs is 1, assume there are no requests to get/put this refs, and we firstly register 10 files and later update these 10 files, and no memory allocations fails, then above two line of codes in __io_sqe_files_update() will be called, before entering percpu_ref_switch_to_atomic(), the count of refs is still one, and |--> percpu_ref_switch_to_atomic |----> __percpu_ref_switch_mode |------> __percpu_ref_switch_to_atomic |-------- > percpu_ref_get(ref), # now the count of refs will be 2. a while later |--> percpu_ref_switch_to_atomic_rcu |----> percpu_ref_call_confirm_rcu |------ > confirm_switch(), # calls io_atomic_switch, note that the count of refs is 2. |------ > percpu_ref_put # drop one ref static void io_atomic_switch(struct percpu_ref *ref) { struct fixed_file_data *data; /* * Juggle reference to ensure we hit zero, if needed, so we can * switch back to percpu mode */ data = container_of(ref, struct fixed_file_data, refs); percpu_ref_put(&data->refs); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ After this operation, the count of refs is 1 now, still not zero, so io_file_data_ref_zero won't be called, then io_ring_file_ref_flush() won't be called, this fixed_file_data's refs will always be in atomic mode, which is bad. percpu_ref_get(&data->refs); } To confirm this bug, I did a hack to kernel: --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -5812,7 +5812,10 @@ static bool io_queue_file_removal(struct fixed_file_data *data, * If we fail allocating the struct we need for doing async reomval * of this file, just punt to sync and wait for it. */ + /* pfile = kzalloc(sizeof(*pfile), GFP_KERNEL); + */ + pfile = NULL; if (!pfile) { pfile = &pfile_stack; pfile->done = &done; To simulate memory allocation failures, then run liburing/test/file-update, [lege@localhost test]$ sudo cat /proc/2091/stack [sudo] password for lege: [<0>] __io_sqe_files_update.isra.85+0x175/0x330 [<0>] __io_uring_register+0x178/0xe20 [<0>] __x64_sys_io_uring_register+0xa0/0x160 [<0>] do_syscall_64+0x55/0x1b0 [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9 (gdb) list * __io_sqe_files_update+0x175 0xffffffff812ec255 is in __io_sqe_files_update (fs/io_uring.c:5830). 5825 llist_add(&pfile->llist, &data->put_llist); 5826 5827 if (pfile == &pfile_stack) { 5828 percpu_ref_switch_to_atomic(&data->refs, io_atomic_switch); 5829 wait_for_completion(&done); 5830 flush_work(&data->ref_work); 5831 return false; file-update will always hang in wait_for_completion(&done), it's because io_ring_file_ref_flush never has a chance to run. I think how to fix this issue a while, doesn't find a elegant method yet. And applications may issue requests continuously, then fixed_file_data's refs may never have a chance to reach zero, refs will always be in atomic mode. Or the simplest method is to use percpu_ref per registered file :) Regards, Xiaoguang Wang Regards, Xiaoguang Wang