Author: kib
Date: Sun Nov 27 09:10:33 2016
New Revision: 309207
URL: https://svnweb.freebsd.org/changeset/base/309207

Log:
  MFC r308618:
  Provide simple mutual exclusion between mount point update and unmount.
  In the update path in ffs_mount(), drop vfs_busy() reference around namei().

Modified:
  stable/11/sys/kern/vfs_mount.c
  stable/11/sys/ufs/ffs/ffs_vfsops.c
Directory Properties:
  stable/11/   (props changed)

Modified: stable/11/sys/kern/vfs_mount.c
==============================================================================
--- stable/11/sys/kern/vfs_mount.c      Sun Nov 27 05:59:17 2016        
(r309206)
+++ stable/11/sys/kern/vfs_mount.c      Sun Nov 27 09:10:33 2016        
(r309207)
@@ -934,6 +934,11 @@ 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);
@@ -1293,6 +1298,7 @@ dounmount(struct mount *mp, int flags, s
        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);

Modified: stable/11/sys/ufs/ffs/ffs_vfsops.c
==============================================================================
--- stable/11/sys/ufs/ffs/ffs_vfsops.c  Sun Nov 27 05:59:17 2016        
(r309206)
+++ stable/11/sys/ufs/ffs/ffs_vfsops.c  Sun Nov 27 09:10:33 2016        
(r309207)
@@ -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;
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