diff options
Diffstat (limited to 'sys/kern/vfs_subr.c')
| -rw-r--r-- | sys/kern/vfs_subr.c | 34 |
1 files changed, 33 insertions, 1 deletions
diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c index 325ca99..3d8152e 100644 --- a/sys/kern/vfs_subr.c +++ b/sys/kern/vfs_subr.c @@ -348,6 +348,38 @@ SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_FIRST, vntblinit, NULL); /* * Mark a mount point as busy. Used to synchronize access and to delay * unmounting. Eventually, mountlist_mtx is not released on failure. + * + * vfs_busy() is a custom lock, it can block the caller. + * 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. */ int vfs_busy(struct mount *mp, int flags) @@ -3022,7 +3054,7 @@ vfs_sysctl(SYSCTL_HANDLER_ARGS) struct vfsconf *vfsp; struct xvfsconf xvfsp; - printf("WARNING: userland calling deprecated sysctl, " + log(LOG_WARNING, "userland calling deprecated sysctl, " "please rebuild world\n"); #if 1 || defined(COMPAT_PRELITE2) |
