When softdep_fsync() is running, a caller must already started write for the mount point. Since unmount or remount to ro suspends mount point, it cannot run in parallel with softdep_fsync(), which makes vfs_busy() call there not needed. Doing blocking vfs_busy() there effectively causes lock order reversal between vn_start_write() and setting MNTK_UNMOUNT, because vfs_busy(mp, 0) sleeps waiting for MNTK_UNMOUNT becoming clear, while unmount sets the flag and starts the suspension. Note that all other uses of vfs_busy() in SU code are non-blocking. Reported by: chs by mckusick Reviewed by: mckusick Tested by: pho Sponsored by: The FreeBSD Foundation MFC after: 1 week |
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