Generalize the VV_CROSSLOCK logic in vfs_lookup()
When VV_CROSSLOCK is present, the lock for the vnode at the current stage of lookup must be held across the VFS_ROOT() call for the filesystem mounted at the vnode. Since VV_CROSSLOCK implies that the root vnode reuses the already-held lock, the possibility for recursion should be made clear in the flags passed to VFS_ROOT(). For cases in which the lock is held exclusive, this means passing LK_CANRECURSE. For cases in which the lock is held shared, it means clearing LK_NODDLKTREAT to allow VFS_ROOT() to potentially recurse on the shared lock even in the presence of an exclusive waiter. That the existing code works for unionfs is due to a coincidence of the current unionfs implementation. Reviewed by: kib Tested by: pho Differential Revision: https://reviews.freebsd.org/D37458
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@ -1265,14 +1265,28 @@ vfs_lookup(struct nameidata *ndp)
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crosslock = (dp->v_vflag & VV_CROSSLOCK) != 0;
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crosslkflags = compute_cn_lkflags(mp, cnp->cn_lkflags,
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cnp->cn_flags);
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if (__predict_false(crosslock) &&
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(crosslkflags & LK_EXCLUSIVE) != 0 &&
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VOP_ISLOCKED(dp) != LK_EXCLUSIVE) {
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vn_lock(dp, LK_UPGRADE | LK_RETRY);
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if (VN_IS_DOOMED(dp)) {
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error = ENOENT;
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goto bad2;
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}
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if (__predict_false(crosslock)) {
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/*
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* We are going to be holding the vnode lock, which
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* in this case is shared by the root vnode of the
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* filesystem mounted at mp, across the call to
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* VFS_ROOT(). Make the situation clear to the
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* filesystem by passing LK_CANRECURSE if the
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* lock is held exclusive, or by clearinng
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* LK_NODDLKTREAT to allow recursion on the shared
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* lock in the presence of an exclusive waiter.
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*/
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if (VOP_ISLOCKED(dp) == LK_EXCLUSIVE) {
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crosslkflags &= ~LK_SHARED;
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crosslkflags |= LK_EXCLUSIVE | LK_CANRECURSE;
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} else if ((crosslkflags & LK_EXCLUSIVE) != 0) {
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vn_lock(dp, LK_UPGRADE | LK_RETRY);
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if (VN_IS_DOOMED(dp)) {
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error = ENOENT;
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goto bad2;
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}
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} else
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crosslkflags &= ~LK_NODDLKTREAT;
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}
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if (vfs_busy(mp, 0) != 0)
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continue;
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