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Tue, 24 Sep 2019 14:07:02 +0000 (UTC) Subject: Re: [PATCH] Btrfs: fix race setting up and completing qgroup rescan workers To: fdmanana@kernel.org, linux-btrfs@vger.kernel.org References: <20190924094954.1304-1-fdmanana@kernel.org> 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: Date: Tue, 24 Sep 2019 17:07:01 +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: <20190924094954.1304-1-fdmanana@kernel.org> 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 24.09.19 г. 12:49 ч., fdmanana@kernel.org wrote: > From: Filipe Manana > > There is a race between setting up a qgroup rescan worker and completing > a qgroup rescan worker that can lead to callers of the qgroup rescan wait > ioctl to either not wait for the rescan worker to complete or to hang > forever due to missing wake ups. The following diagram shows a sequence > of steps that illustrates the race. > > CPU 1 CPU 2 CPU 3 > > btrfs_ioctl_quota_rescan() > btrfs_qgroup_rescan() > qgroup_rescan_init() > mutex_lock(&fs_info->qgroup_rescan_lock) > spin_lock(&fs_info->qgroup_lock) > > fs_info->qgroup_flags |= > BTRFS_QGROUP_STATUS_FLAG_RESCAN > > init_completion( > &fs_info->qgroup_rescan_completion) > > fs_info->qgroup_rescan_running = true > > mutex_unlock(&fs_info->qgroup_rescan_lock) > spin_unlock(&fs_info->qgroup_lock) > > btrfs_init_work() > --> starts the worker > > btrfs_qgroup_rescan_worker() > mutex_lock(&fs_info->qgroup_rescan_lock) > > fs_info->qgroup_flags &= > ~BTRFS_QGROUP_STATUS_FLAG_RESCAN > > mutex_unlock(&fs_info->qgroup_rescan_lock) > > starts transaction, updates qgroup status > item, etc > > btrfs_ioctl_quota_rescan() > btrfs_qgroup_rescan() > qgroup_rescan_init() > mutex_lock(&fs_info->qgroup_rescan_lock) > spin_lock(&fs_info->qgroup_lock) > > fs_info->qgroup_flags |= > BTRFS_QGROUP_STATUS_FLAG_RESCAN > > init_completion( > &fs_info->qgroup_rescan_completion) > > fs_info->qgroup_rescan_running = true > > mutex_unlock(&fs_info->qgroup_rescan_lock) > spin_unlock(&fs_info->qgroup_lock) > > btrfs_init_work() > --> starts another worker > > mutex_lock(&fs_info->qgroup_rescan_lock) > > fs_info->qgroup_rescan_running = false > > mutex_unlock(&fs_info->qgroup_rescan_lock) > > complete_all(&fs_info->qgroup_rescan_completion) > > Before the rescan worker started by the task at CPU 3 completes, if another > task calls btrfs_ioctl_quota_rescan(), it will get -EINPROGRESS because the > flag BTRFS_QGROUP_STATUS_FLAG_RESCAN is set at fs_info->qgroup_flags, which > is expected and correct behaviour. > > However if other task calls btrfs_ioctl_quota_rescan_wait() before the > rescan worker started by the task at CPU 3 completes, it will return > immediately without waiting for the new rescan worker to complete, > because fs_info->qgroup_rescan_running is set to false by CPU 2. > > This race is making test case btrfs/171 (from fstests) to fail often: > > btrfs/171 9s ... - output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/171.out.bad) > --- tests/btrfs/171.out 2018-09-16 21:30:48.505104287 +0100 > +++ /home/fdmanana/git/hub/xfstests/results//btrfs/171.out.bad 2019-09-19 02:01:36.938486039 +0100 > @@ -1,2 +1,3 @@ > QA output created by 171 > +ERROR: quota rescan failed: Operation now in progress > Silence is golden > ... > (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/171.out /home/fdmanana/git/hub/xfstests/results//btrfs/171.out.bad' to see the entire diff) > > That is because the test calls the btrfs-progs commands "qgroup quota > rescan -w", "qgroup assign" and "qgroup remove" in a sequence that makes > calls to the rescan start ioctl fail with -EINPROGRESS (note the "btrfs" > commands 'qgroup assign' and 'qgroup remove' often call the rescan start > ioctl after calling the qgroup assign ioctl, btrfs_ioctl_qgroup_assign()), > since previous waits didn't actually wait for a rescan worker to complete. > > Another problem the race can cause is missing wake ups for waiters, since > the call to complete_all() happens outside a critical section and after > clearing the flag BTRFS_QGROUP_STATUS_FLAG_RESCAN. In the sequence diagram > above, if we have a waiter for the first rescan task (executed by CPU 2), > then fs_info->qgroup_rescan_completion.wait is not empty, and if after the > rescan worker clears BTRFS_QGROUP_STATUS_FLAG_RESCAN and before it calls > complete_all() against fs_info->qgroup_rescan_completion, the task at CPU 3 > calls init_completion() against fs_info->qgroup_rescan_completion which > re-initilizes its wait queue to an empty queue, therefore causing the > rescan worker at CPU 2 to call complete_all() against an empty queue, never > waking up the task waiting for that rescan worker. > > Fix this by clearing BTRFS_QGROUP_STATUS_FLAG_RESCAN and setting > fs_info->qgroup_rescan_running to false in the same critical section, > delimited by the mutex fs_info->qgroup_rescan_lock, as well as doing Why do we need both the RESCAN flag and qgroup_rescan_running having them both and having btrfs_qgroup_wait_for_completion rely on qgroup[_rescan_running just makes it easier to screw up. IMO it makes sense to remove qgroup_rescan_running in this patch as well and the code should only use RESCAN flag.