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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 6E9A1C433FF for ; Wed, 7 Aug 2019 07:17:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 406562199C for ; Wed, 7 Aug 2019 07:17:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727428AbfHGHR3 (ORCPT ); Wed, 7 Aug 2019 03:17:29 -0400 Received: from mx2.suse.de ([195.135.220.15]:54616 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727281AbfHGHR3 (ORCPT ); Wed, 7 Aug 2019 03:17:29 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 9B1C9AF40; Wed, 7 Aug 2019 07:17:27 +0000 (UTC) Subject: Re: [PATCH] Btrfs: fix workqueue deadlock on dependent filesystems To: Omar Sandoval , linux-btrfs@vger.kernel.org Cc: kernel-team@fb.com, Tejun Heo References: <0bea516a54b26e4e1c42e6fe47548cb48cc4172b.1565112813.git.osandov@fb.com> From: Nikolay Borisov Openpgp: preference=signencrypt Autocrypt: addr=nborisov@suse.com; prefer-encrypt=mutual; keydata= mQINBFiKBz4BEADNHZmqwhuN6EAzXj9SpPpH/nSSP8YgfwoOqwrP+JR4pIqRK0AWWeWCSwmZ T7g+RbfPFlmQp+EwFWOtABXlKC54zgSf+uulGwx5JAUFVUIRBmnHOYi/lUiE0yhpnb1KCA7f u/W+DkwGerXqhhe9TvQoGwgCKNfzFPZoM+gZrm+kWv03QLUCr210n4cwaCPJ0Nr9Z3c582xc bCUVbsjt7BN0CFa2BByulrx5xD9sDAYIqfLCcZetAqsTRGxM7LD0kh5WlKzOeAXj5r8DOrU2 GdZS33uKZI/kZJZVytSmZpswDsKhnGzRN1BANGP8sC+WD4eRXajOmNh2HL4P+meO1TlM3GLl EQd2shHFY0qjEo7wxKZI1RyZZ5AgJnSmehrPCyuIyVY210CbMaIKHUIsTqRgY5GaNME24w7h TyyVCy2qAM8fLJ4Vw5bycM/u5xfWm7gyTb9V1TkZ3o1MTrEsrcqFiRrBY94Rs0oQkZvunqia c+NprYSaOG1Cta14o94eMH271Kka/reEwSZkC7T+o9hZ4zi2CcLcY0DXj0qdId7vUKSJjEep c++s8ncFekh1MPhkOgNj8pk17OAESanmDwksmzh1j12lgA5lTFPrJeRNu6/isC2zyZhTwMWs k3LkcTa8ZXxh0RfWAqgx/ogKPk4ZxOXQEZetkEyTFghbRH2BIwARAQABtCNOaWtvbGF5IEJv cmlzb3YgPG5ib3Jpc292QHN1c2UuY29tPokCOAQTAQIAIgUCWIo48QIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQcb6CRuU/KFc0eg/9GLD3wTQz9iZHMFbjiqTCitD7B6dTLV1C ddZVlC8Hm/TophPts1bWZORAmYIihHHI1EIF19+bfIr46pvfTu0yFrJDLOADMDH+Ufzsfy2v HSqqWV/nOSWGXzh8bgg/ncLwrIdEwBQBN9SDS6aqsglagvwFD91UCg/TshLlRxD5BOnuzfzI Leyx2c6YmH7Oa1R4MX9Jo79SaKwdHt2yRN3SochVtxCyafDlZsE/efp21pMiaK1HoCOZTBp5 VzrIP85GATh18pN7YR9CuPxxN0V6IzT7IlhS4Jgj0NXh6vi1DlmKspr+FOevu4RVXqqcNTSS E2rycB2v6cttH21UUdu/0FtMBKh+rv8+yD49FxMYnTi1jwVzr208vDdRU2v7Ij/TxYt/v4O8 V+jNRKy5Fevca/1xroQBICXsNoFLr10X5IjmhAhqIH8Atpz/89ItS3+HWuE4BHB6RRLM0gy8 T7rN6ja+KegOGikp/VTwBlszhvfLhyoyjXI44Tf3oLSFM+8+qG3B7MNBHOt60CQlMkq0fGXd mm4xENl/SSeHsiomdveeq7cNGpHi6i6ntZK33XJLwvyf00PD7tip/GUj0Dic/ZUsoPSTF/mG EpuQiUZs8X2xjK/AS/l3wa4Kz2tlcOKSKpIpna7V1+CMNkNzaCOlbv7QwprAerKYywPCoOSC 7P25Ag0EWIoHPgEQAMiUqvRBZNvPvki34O/dcTodvLSyOmK/MMBDrzN8Cnk302XfnGlW/YAQ csMWISKKSpStc6tmD+2Y0z9WjyRqFr3EGfH1RXSv9Z1vmfPzU42jsdZn667UxrRcVQXUgoKg QYx055Q2FdUeaZSaivoIBD9WtJq/66UPXRRr4H/+Y5FaUZx+gWNGmBT6a0S/GQnHb9g3nonD jmDKGw+YO4P6aEMxyy3k9PstaoiyBXnzQASzdOi39BgWQuZfIQjN0aW+Dm8kOAfT5i/yk59h VV6v3NLHBjHVw9kHli3jwvsizIX9X2W8tb1SefaVxqvqO1132AO8V9CbE1DcVT8fzICvGi42 FoV/k0QOGwq+LmLf0t04Q0csEl+h69ZcqeBSQcIMm/Ir+NorfCr6HjrB6lW7giBkQl6hhomn l1mtDP6MTdbyYzEiBFcwQD4terc7S/8ELRRybWQHQp7sxQM/Lnuhs77MgY/e6c5AVWnMKd/z MKm4ru7A8+8gdHeydrRQSWDaVbfy3Hup0Ia76J9FaolnjB8YLUOJPdhI2vbvNCQ2ipxw3Y3c KhVIpGYqwdvFIiz0Fej7wnJICIrpJs/+XLQHyqcmERn3s/iWwBpeogrx2Lf8AGezqnv9woq7 OSoWlwXDJiUdaqPEB/HmGfqoRRN20jx+OOvuaBMPAPb+aKJyle8zABEBAAGJAh8EGAECAAkF AliKBz4CGwwACgkQcb6CRuU/KFdacg/+M3V3Ti9JYZEiIyVhqs+yHb6NMI1R0kkAmzsGQ1jU zSQUz9AVMR6T7v2fIETTT/f5Oout0+Hi9cY8uLpk8CWno9V9eR/B7Ifs2pAA8lh2nW43FFwp IDiSuDbH6oTLmiGCB206IvSuaQCp1fed8U6yuqGFcnf0ZpJm/sILG2ECdFK9RYnMIaeqlNQm iZicBY2lmlYFBEaMXHoy+K7nbOuizPWdUKoKHq+tmZ3iA+qL5s6Qlm4trH28/fPpFuOmgP8P K+7LpYLNSl1oQUr+WlqilPAuLcCo5Vdl7M7VFLMq4xxY/dY99aZx0ZJQYFx0w/6UkbDdFLzN upT7NIN68lZRucImffiWyN7CjH23X3Tni8bS9ubo7OON68NbPz1YIaYaHmnVQCjDyDXkQoKC R82Vf9mf5slj0Vlpf+/Wpsv/TH8X32ajva37oEQTkWNMsDxyw3aPSps6MaMafcN7k60y2Wk/ TCiLsRHFfMHFY6/lq/c0ZdOsGjgpIK0G0z6et9YU6MaPuKwNY4kBdjPNBwHreucrQVUdqRRm RcxmGC6ohvpqVGfhT48ZPZKZEWM+tZky0mO7bhZYxMXyVjBn4EoNTsXy1et9Y1dU3HVJ8fod 5UqrNrzIQFbdeM0/JqSLrtlTcXKJ7cYFa9ZM2AP7UIN9n1UWxq+OPY9YMOewVfYtL8M= Message-ID: <70f6b4fa-26b1-9225-7509-aa89bb7e067c@suse.com> Date: Wed, 7 Aug 2019 10:17:26 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <0bea516a54b26e4e1c42e6fe47548cb48cc4172b.1565112813.git.osandov@fb.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 6.08.19 г. 20:34 ч., Omar Sandoval wrote: > From: Omar Sandoval > > We hit a the following very strange deadlock on a system with Btrfs on a > loop device backed by another Btrfs filesystem: > > 1. The top (loop device) filesystem queues an async_cow work item from > cow_file_range_async(). We'll call this work X. > 2. Worker thread A starts work X (normal_work_helper()). > 3. Worker thread A executes the ordered work for the top filesystem > (run_ordered_work()). > 4. Worker thread A finishes the ordered work for work X and frees X > (work->ordered_free()). > 5. Worker thread A executes another ordered work and gets blocked on I/O > to the bottom filesystem (still in run_ordered_work()). > 6. Meanwhile, the bottom filesystem allocates and queues an async_cow > work item which happens to be the recently-freed X. > 7. The workqueue code sees that X is already being executed by worker > thread A, so it schedules X to be executed _after_ worker thread A > finishes (see the find_worker_executing_work() call in > process_one_work()). Isn't the bigger problem that a single run_ordered_work could potentially run the ordered work for more than one normal work? E.g. what if btrfs' code is reworked such that run_ordered_work executes ordered_func for just one work item (the one which called the function in the first place) ? Wouldn't that also resolve the issue? Correct me if I'm wrong but it seems silly to have one work item outlive ordered_free which is what currently happens, right? > > Now, the top filesystem is waiting for I/O on the bottom filesystem, but > the bottom filesystem is waiting for the top filesystem to finish, so we > deadlock. > > This happens because we are breaking the workqueue assumption that a > work item cannot be recycled while it still depends on other work. Fix > it by waiting to free the work item until we are done with all of the > related ordered work. > > P.S.: > > One might ask why the workqueue code doesn't try to detect a recycled > work item. It actually does try by checking whether the work item has > the same work function (find_worker_executing_work()), but in our case > the function is the same. This is the only key that the workqueue code > has available to compare, short of adding an additional, layer-violating > "custom key". Considering that we're the only ones that have ever hit > this, we should just play by the rules. > > Unfortunately, we haven't been able to create a minimal reproducer other > than our full container setup using a compress-force=zstd filesystem on > top of another compress-force=zstd filesystem. > > Suggested-by: Tejun Heo > Signed-off-by: Omar Sandoval > --- > fs/btrfs/async-thread.c | 56 ++++++++++++++++++++++++++++++++--------- > 1 file changed, 44 insertions(+), 12 deletions(-) > > diff --git a/fs/btrfs/async-thread.c b/fs/btrfs/async-thread.c > index 122cb97c7909..b2bfde560331 100644 > --- a/fs/btrfs/async-thread.c > +++ b/fs/btrfs/async-thread.c > @@ -250,16 +250,17 @@ static inline void thresh_exec_hook(struct __btrfs_workqueue *wq) > } > } > > -static void run_ordered_work(struct __btrfs_workqueue *wq) > +static void run_ordered_work(struct btrfs_work *self) > { > + struct __btrfs_workqueue *wq = self->wq; > struct list_head *list = &wq->ordered_list; > struct btrfs_work *work; > spinlock_t *lock = &wq->list_lock; > unsigned long flags; > + void *wtag; > + bool free_self = false; > > while (1) { > - void *wtag; > - > spin_lock_irqsave(lock, flags); > if (list_empty(list)) > break; > @@ -285,16 +286,47 @@ static void run_ordered_work(struct __btrfs_workqueue *wq) > list_del(&work->ordered_list); > spin_unlock_irqrestore(lock, flags); > > - /* > - * We don't want to call the ordered free functions with the > - * lock held though. Save the work as tag for the trace event, > - * because the callback could free the structure. > - */ > - wtag = work; > - work->ordered_free(work); > - trace_btrfs_all_work_done(wq->fs_info, wtag); > + if (work == self) { > + /* > + * This is the work item that the worker is currently > + * executing. > + * > + * The kernel workqueue code guarantees non-reentrancy > + * of work items. I.e., if a work item with the same > + * address and work function is queued twice, the second > + * execution is blocked until the first one finishes. A > + * work item may be freed and recycled with the same > + * work function; the workqueue code assumes that the > + * original work item cannot depend on the recycled work > + * item in that case (see find_worker_executing_work()). > + * > + * Note that the work of one Btrfs filesystem may depend > + * on the work of another Btrfs filesystem via, e.g., a > + * loop device. Therefore, we must not allow the current > + * work item to be recycled until we are really done, > + * otherwise we break the above assumption and can > + * deadlock. > + */ > + free_self = true; > + } else { > + /* > + * We don't want to call the ordered free functions with > + * the lock held though. Save the work as tag for the > + * trace event, because the callback could free the > + * structure. > + */ > + wtag = work; > + work->ordered_free(work); > + trace_btrfs_all_work_done(wq->fs_info, wtag); > + } > } > spin_unlock_irqrestore(lock, flags); > + > + if (free_self) { > + wtag = self; > + self->ordered_free(self); > + trace_btrfs_all_work_done(wq->fs_info, wtag); > + } > } > > static void normal_work_helper(struct btrfs_work *work) > @@ -322,7 +354,7 @@ static void normal_work_helper(struct btrfs_work *work) > work->func(work); > if (need_order) { > set_bit(WORK_DONE_BIT, &work->flags); > - run_ordered_work(wq); > + run_ordered_work(work); > } > if (!need_order) > trace_btrfs_all_work_done(wq->fs_info, wtag); >