Jason A. Harmening
080ef8a418
When a lookup operation crosses into a new mountpoint, the mountpoint must first be busied before the root vnode can be locked. When a filesystem is unmounted, the vnode covered by the mountpoint must first be locked, and then the busy count for the mountpoint drained. Ordinarily, these two operations work fine if executed concurrently, but with a stacked filesystem the root vnode may in fact use the same lock as the covered vnode. By design, this will always be the case for unionfs (with either the upper or lower root vnode depending on mount options), and can also be the case for nullfs if the target and mount point are the same (which admittedly is very unlikely in practice). In this case, we have LOR. The lookup path holds the mountpoint busy while waiting on what is effectively the covered vnode lock, while a concurrent unmount holds the covered vnode lock and waits for the mountpoint's busy count to drain. Attempt to resolve this LOR by allowing the stacked filesystem to specify a new flag, VV_CROSSLOCK, on a covered vnode as necessary. Upon observing this flag, the vfs_lookup() will leave the covered vnode lock held while crossing into the mountpoint. Employ this flag for unionfs with the caveat that it can't be used for '-o below' mounts until other unionfs locking issues are resolved. Reported by: pho Tested by: pho Reviewed by: kib Differential Revision: https://reviews.freebsd.org/D35054
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