On Mon, Jul 08, 2013 at 12:26:43AM +0200, Andreas Longwitz wrote: > The deadlock can be explained now: pid 1 (init) sleeps on "mount drain" > because mp->mnt_lockref was 1. This setting was done by pid 18 (gjournal > switcher) by calling vfs_busy(). pid 18 now sleeps on "suspwt" because > mp->mnt_writeopcount was 1. This setting was done by pid 1 before going > to sleep by calling vn_start_write() in dounmount(). > > I think the reason for this deadlock is the commit r249055 which seems > not to be compatible with gjournal. Thank you for the analysis. I think 'not compatible' is some understatement. The situation clearly causes a deadlock, you are right.
The vfs_busy(); vfs_write_suspend(); call sequence is somewhat dubious,
in fact, exactly because unmount could start in between. I think that
vfs_write_suspend() must avoid setting MNT_SUSPEND if unmount was
started. Patch below, for HEAD, should fix the problem, by marking the
callers of vfs_write_suspend(), which are not protected by the covered
vnode lock, with the VS_SKIP_UNMOUNT flag.
I believe that the conflicts on stable/8 should be trivial, if any.
diff --git a/sys/geom/journal/g_journal.c b/sys/geom/journal/g_journal.c
index a3c996c..3ce2785 100644
--- a/sys/geom/journal/g_journal.c
+++ b/sys/geom/journal/g_journal.c
@@ -2960,7 +2960,7 @@ g_journal_do_switch(struct g_class *classp)
GJ_TIMER_STOP(1, &bt, "BIO_FLUSH time of %s", sc->sc_name);
GJ_TIMER_START(1, &bt);
- error = vfs_write_suspend(mp);
+ error = vfs_write_suspend(mp, VS_SKIP_UNMOUNT);
GJ_TIMER_STOP(1, &bt, "Suspend time of %s", mountpoint);
if (error != 0) {
GJ_DEBUG(0, "Cannot suspend file system %s (error=%d).",
diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index 7eac0ef..06e59f9 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -1668,8 +1668,7 @@ vn_finished_secondary_write(mp)
* Request a filesystem to suspend write operations.
*/
int
-vfs_write_suspend(mp)
- struct mount *mp;
+vfs_write_suspend(struct mount *mp, int flags)
{
int error;
@@ -1680,6 +1679,21 @@ vfs_write_suspend(mp)
}
while (mp->mnt_kern_flag & MNTK_SUSPEND)
msleep(&mp->mnt_flag, MNT_MTX(mp), PUSER - 1, "wsuspfs", 0);
+
+ /*
+ * Unmount holds a write reference on the mount point. If we
+ * own busy reference and drain for writers, we deadlock with
+ * the reference draining in the unmount path. Callers of
+ * vfs_write_suspend() must specify VS_SKIP_UNMOUNT if
+ * vfs_busy() reference is owned and caller is not in the
+ * unmount context.
+ */
+ if ((flags & VS_SKIP_UNMOUNT) != 0 &&
+ (mp->mnt_kern_flag & MNTK_UNMOUNT) != 0) {
+ MNT_IUNLOCK(mp);
+ return (EBUSY);
+ }
+
mp->mnt_kern_flag |= MNTK_SUSPEND;
mp->mnt_susp_owner = curthread;
if (mp->mnt_writeopcount > 0)
diff --git a/sys/sys/vnode.h b/sys/sys/vnode.h
index 42bfb65..b0cbcc0 100644
--- a/sys/sys/vnode.h
+++ b/sys/sys/vnode.h
@@ -398,6 +398,9 @@ extern int vttoif_tab[];
#define VR_START_WRITE 0x0001 /* vfs_write_resume: start write
atomically */
#define VR_NO_SUSPCLR 0x0002 /* vfs_write_resume: do not clear
suspension */
+#define VS_SKIP_UNMOUNT 0x0001 /* vfs_write_suspend: fail if the
+ filesystem is being unmounted */
+
#define VREF(vp) vref(vp)
#ifdef DIAGNOSTIC
@@ -711,7 +714,7 @@ int vn_io_fault_pgmove(vm_page_t ma[], vm_offset_t offset,
int xfersize,
int vfs_cache_lookup(struct vop_lookup_args *ap);
void vfs_timestamp(struct timespec *);
void vfs_write_resume(struct mount *mp, int flags);
-int vfs_write_suspend(struct mount *mp);
+int vfs_write_suspend(struct mount *mp, int flags);
int vop_stdbmap(struct vop_bmap_args *);
int vop_stdfsync(struct vop_fsync_args *);
int vop_stdgetwritemount(struct vop_getwritemount_args *);
diff --git a/sys/ufs/ffs/ffs_snapshot.c b/sys/ufs/ffs/ffs_snapshot.c
index 9a9c88a..ad157aa 100644
--- a/sys/ufs/ffs/ffs_snapshot.c
+++ b/sys/ufs/ffs/ffs_snapshot.c
@@ -423,7 +423,7 @@ restart:
*/
for (;;) {
vn_finished_write(wrtmp);
- if ((error = vfs_write_suspend(vp->v_mount)) != 0) {
+ if ((error = vfs_write_suspend(vp->v_mount, 0)) != 0) {
vn_start_write(NULL, &wrtmp, V_WAIT);
vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
goto out;
diff --git a/sys/ufs/ffs/ffs_suspend.c b/sys/ufs/ffs/ffs_suspend.c
index 3198c1a..a8c4578 100644
--- a/sys/ufs/ffs/ffs_suspend.c
+++ b/sys/ufs/ffs/ffs_suspend.c
@@ -206,7 +206,7 @@ ffs_susp_suspend(struct mount *mp)
return (EPERM);
#endif
- if ((error = vfs_write_suspend(mp)) != 0)
+ if ((error = vfs_write_suspend(mp, VS_SKIP_UNMOUNT)) != 0)
return (error);
ump->um_writesuspended = 1;
diff --git a/sys/ufs/ffs/ffs_vfsops.c b/sys/ufs/ffs/ffs_vfsops.c
index 57f092c..a87fdfa 100644
--- a/sys/ufs/ffs/ffs_vfsops.c
+++ b/sys/ufs/ffs/ffs_vfsops.c
@@ -257,7 +257,7 @@ ffs_mount(struct mount *mp)
return (error);
for (;;) {
vn_finished_write(mp);
- if ((error = vfs_write_suspend(mp)) != 0)
+ if ((error = vfs_write_suspend(mp, 0)) != 0)
return (error);
MNT_ILOCK(mp);
if (mp->mnt_kern_flag & MNTK_SUSPENDED) {
@@ -1255,7 +1255,7 @@ ffs_unmount(mp, mntflags)
*/
for (;;) {
vn_finished_write(mp);
- if ((error = vfs_write_suspend(mp)) != 0)
+ if ((error = vfs_write_suspend(mp, 0)) != 0)
return (error);
MNT_ILOCK(mp);
if (mp->mnt_kern_flag & MNTK_SUSPENDED) {
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