svn commit: r226259 - stable/7/sys/kern
kib at FreeBSD.org
Tue Oct 11 14:05:40 UTC 2011
Date: Tue Oct 11 14:05:39 2011
New Revision: 226259
Move parts of the commit log for r166167, where Tor explained the
interaction between vnode locks and vfs_busy(), into comment.
Comment is slightly edited to better fit into RELENG_7 reality.
stable/7/sys/ (props changed)
stable/7/sys/cddl/contrib/opensolaris/ (props changed)
stable/7/sys/contrib/dev/acpica/ (props changed)
stable/7/sys/contrib/pf/ (props changed)
--- stable/7/sys/kern/vfs_subr.c Tue Oct 11 14:03:18 2011 (r226258)
+++ stable/7/sys/kern/vfs_subr.c Tue Oct 11 14:05:39 2011 (r226259)
@@ -335,6 +335,37 @@ SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_FIRST,
* Mark a mount point as busy. Used to synchronize access and to delay
* unmounting. Interlock is not released on failure.
+ * vfs_busy() only sleeps if the unmount is active on the mount point.
+ * For a mountpoint mp, vfs_busy-enforced lock is before lock of any
+ * vnode belonging to mp.
+ * Lookup uses vfs_busy() to traverse mount points.
+ * root fs var fs
+ * / vnode lock A / vnode lock (/var) D
+ * /var vnode lock B /log vnode lock(/var/log) E
+ * vfs_busy lock C vfs_busy lock F
+ * Within each file system, the lock order is C->A->B and F->D->E.
+ * When traversing across mounts, the system follows that lock order:
+ * C->A->B
+ * |
+ * +->F->D->E
+ * The lookup() process for namei("/var") illustrates the process:
+ * VOP_LOOKUP() obtains B while A is held
+ * vfs_busy() obtains a shared lock on F while A and B are held
+ * vput() releases lock on B
+ * vput() releases lock on A
+ * VFS_ROOT() obtains lock on D while shared lock on F is held
+ * vfs_unbusy() releases shared lock on F
+ * vn_lock() obtains lock on deadfs vnode vp_crossmp instead of A.
+ * Attempt to lock A (instead of vp_crossmp) while D is held would
+ * violate the global order, causing deadlocks.
+ * dounmount() locks B while F is drained.
vfs_busy(struct mount *mp, int flags, struct mtx *interlkp,
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