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Provide simple mutual exclusion between mount point update and unmount.

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Currently mount update keeps vfs_busy(9) reference on the mount point
during MNT_UPDATE VFS_MOUNT() vfsops call.  This already provides the
exclusion, but is problematic for filesystems which need to perform
namei(9) during VFS_MOUNT(MNT_UPDATE) operations, e.g. to refresh
mnt_from path, because namei(9) must not be called while the
vfs_busy(9) reference is owned.

Check for MNT_UPDATE flag before setting MNTK_UNMOUNT, and for
MNTK_UNMOUNT before entering innards of vfs_domount_update(), failing
syscalls with EBUSY if conflict is detected.  Keep vfs_busy(9)
reference around VFS_MOUNT(MNT_UPDATE) calls still to not change VFS
KPI.

In the update path in ffs_mount(), drop vfs_busy() reference around
namei(), which is now safe due to unmount never executing in parallel
with VFS_MOUNT(MNT_UPDATE), and which avoids the deadlock.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
This commit is contained in:
kib 2016-11-13 21:49:51 +00:00
parent 7a4fd2a8e7
commit aef0061353
2 changed files with 24 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
sys/kern/vfs_mount.c
View File

@ -940,6 +940,11 @@ vfs_domount_update(
VOP_UNLOCK(vp, 0);
MNT_ILOCK(mp);
if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0) {
MNT_IUNLOCK(mp);
error = EBUSY;
goto end;
}
mp->mnt_flag &= ~MNT_UPDATEMASK;
mp->mnt_flag |= fsflags & (MNT_RELOAD | MNT_FORCE | MNT_UPDATE |
MNT_SNAPSHOT | MNT_ROOTFS | MNT_UPDATEMASK | MNT_RDONLY);
@ -1299,6 +1304,7 @@ dounmount(struct mount *mp, int flags, struct thread *td)
vn_start_write(NULL, &mp, V_WAIT | V_MNTREF);
MNT_ILOCK(mp);
if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0 ||
(mp->mnt_flag & MNT_UPDATE) != 0 ||
!TAILQ_EMPTY(&mp->mnt_uppers)) {
dounmount_cleanup(mp, coveredvp, 0);
return (EBUSY);

20
sys/ufs/ffs/ffs_vfsops.c
View File

@ -147,7 +147,7 @@ ffs_mount(struct mount *mp)
struct ufsmount *ump = NULL;
struct fs *fs;
pid_t fsckpid = 0;
int error, flags;
int error, error1, flags;
uint64_t mntorflags;
accmode_t accmode;
struct nameidata ndp;
@ -453,6 +453,11 @@ ffs_mount(struct mount *mp)
*/
if (mp->mnt_flag & MNT_SNAPSHOT)
return (ffs_snapshot(mp, fspec));
/*
* Must not call namei() while owning busy ref.
*/
vfs_unbusy(mp);
}
/*
@ -460,7 +465,18 @@ ffs_mount(struct mount *mp)
* and verify that it refers to a sensible disk device.
*/
NDINIT(&ndp, LOOKUP, FOLLOW | LOCKLEAF, UIO_SYSSPACE, fspec, td);
if ((error = namei(&ndp)) != 0)
error = namei(&ndp);
if ((mp->mnt_flag & MNT_UPDATE) != 0) {
/*
* Unmount does not start if MNT_UPDATE is set. Mount
* update busies mp before setting MNT_UPDATE. We
* must be able to retain our busy ref succesfully,
* without sleep.
*/
error1 = vfs_busy(mp, MBF_NOWAIT);
MPASS(error1 == 0);
}
if (error != 0)
return (error);
NDFREE(&ndp, NDF_ONLY_PNBUF);
devvp = ndp.ni_vp;