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There are several code sequences like

Browse source 
vfs_busy(mp);
      vfs_write_suspend(mp);
which are problematic if other thread starts unmount between two
calls.  The unmount starts a write, while vfs_write_suspend() drain
writers.  On the other hand, unmount drains busy references, causing
the deadlock.

Add a flag argument to vfs_write_suspend and require the callers of it
to specify VS_SKIP_UNMOUNT flag, when the call is performed not in the
mount path, i.e. the covered vnode is not locked.  The suspension is
not attempted if VS_SKIP_UNMOUNT is specified and unmount is in
progress.

Reported and tested by:	Andreas Longwitz <longwitz@incore.de>
Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks
This commit is contained in:
Konstantin Belousov 2013-07-09 20:49:32 +00:00
parent b71f585303
commit cc3d8c35f5
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=253106
6 changed files with 25 additions and 8 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
sys/geom/journal/g_journal.c
View file

@ -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).",

18
sys/kern/vfs_vnops.c
View file

@ -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)

5
sys/sys/vnode.h
View file

@ -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 *);

2
sys/ufs/ffs/ffs_snapshot.c
View file

@ -423,7 +423,7 @@ ffs_snapshot(mp, snapfile)
*/
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;

2
sys/ufs/ffs/ffs_suspend.c
View file

@ -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;

4
sys/ufs/ffs/ffs_vfsops.c
View file

@ -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) {