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Generalize the VV_CROSSLOCK logic in vfs_lookup()

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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
This commit is contained in:
Jason A. Harmening 2022-10-26 17:25:20 -05:00
parent bc2ccf0e4f
commit 42442d7a6e
1 changed files with 22 additions and 8 deletions
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30
sys/kern/vfs_lookup.c
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@ -1265,14 +1265,28 @@ vfs_lookup(struct nameidata *ndp)
crosslock = (dp->v_vflag & VV_CROSSLOCK) != 0;
crosslkflags = compute_cn_lkflags(mp, cnp->cn_lkflags,
cnp->cn_flags);
if (__predict_false(crosslock) &&
(crosslkflags & LK_EXCLUSIVE) != 0 &&
VOP_ISLOCKED(dp) != LK_EXCLUSIVE) {
vn_lock(dp, LK_UPGRADE | LK_RETRY);
if (VN_IS_DOOMED(dp)) {
error = ENOENT;
goto bad2;
}
if (__predict_false(crosslock)) {
/*
* We are going to be holding the vnode lock, which
* in this case is shared by the root vnode of the
* filesystem mounted at mp, across the call to
* VFS_ROOT(). Make the situation clear to the
* filesystem by passing LK_CANRECURSE if the
* lock is held exclusive, or by clearinng
* LK_NODDLKTREAT to allow recursion on the shared
* lock in the presence of an exclusive waiter.
*/
if (VOP_ISLOCKED(dp) == LK_EXCLUSIVE) {
crosslkflags &= ~LK_SHARED;
crosslkflags |= LK_EXCLUSIVE | LK_CANRECURSE;
} else if ((crosslkflags & LK_EXCLUSIVE) != 0) {
vn_lock(dp, LK_UPGRADE | LK_RETRY);
if (VN_IS_DOOMED(dp)) {
error = ENOENT;
goto bad2;
}
} else
crosslkflags &= ~LK_NODDLKTREAT;
}
if (vfs_busy(mp, 0) != 0)
continue;