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The quotaoff operation has a race with inode allocation that results in a livelock. An inode allocation that occurs before the quota status flags are updated acquires the appropriate dquots for the inode via xfs_qm_vop_dqalloc(). It then inserts the XFS_INEW inode into the perag radix tree, sometime later attaches the dquots to the inode and finally clears the XFS_INEW flag. Quotaoff expects to release the dquots from all inodes in the filesystem via xfs_qm_dqrele_all_inodes(). This invokes the AG inode iterator, which skips inodes in the XFS_INEW state because they are not fully constructed. If the scan occurs after dquots have been attached to an inode, but before XFS_INEW is cleared, the newly allocated inode will continue to hold a reference to the applicable dquots. When quotaoff invokes xfs_qm_dqpurge_all(), the reference count of those dquot(s) remain elevated and the dqpurge scan spins indefinitely. To address this problem, update the xfs_qm_dqrele_all_inodes() scan to wait on inodes marked on the XFS_INEW state. We wait on the inodes explicitly rather than skip and retry to avoid continuous retry loops due to a parallel inode allocation workload. Since quotaoff updates the quota state flags and uses a synchronous transaction before the dqrele scan, and dquots are attached to inodes after radix tree insertion iff quota is enabled, one INEW waiting pass through the AG guarantees that the scan has processed all inodes that could possibly hold dquot references. Reported-by: Eryu Guan <eguan@redhat.com> Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.