svn commit: r233629 - head/sys/ufs/ffs

[Kirk McKusick]

Author: mckusick
Date: Wed Mar 28 21:21:19 2012
New Revision: 233629
URL: http://svn.freebsd.org/changeset/base/233629

Log:
  A refinement of change 232351 to avoid a race with a forcible unmount.
  While we have a snapshot vnode unlocked to avoid a deadlock with another
  inode in the same inode block being updated, the filesystem containing
  it may be forcibly unmounted. When that happens the snapshot vnode is
  revoked. We need to check for that condition and fail appropriately.
  
  This change will be included along with 232351 when it is MFC'ed to 9.
  
  Spotted by:  kib
  Reviewed by: kib

Modified:
  head/sys/ufs/ffs/ffs_inode.c

Modified: head/sys/ufs/ffs/ffs_inode.c
==============================================================================
--- head/sys/ufs/ffs/ffs_inode.c	Wed Mar 28 20:58:30 2012	(r233628)
+++ head/sys/ufs/ffs/ffs_inode.c	Wed Mar 28 21:21:19 2012	(r233629)
@@ -106,6 +106,7 @@ ffs_update(vp, waitfor)
 	flags = 0;
 	if (IS_SNAPSHOT(ip))
 		flags = GB_LOCK_NOWAIT;
+loop:
 	error = breadn_flags(ip->i_devvp,
 	     fsbtodb(fs, ino_to_fsba(fs, ip->i_number)),
 	     (int) fs->fs_bsize, 0, 0, 0, NOCRED, flags, &bp);
@@ -115,13 +116,27 @@ ffs_update(vp, waitfor)
 			return (error);
 		}
 		KASSERT((IS_SNAPSHOT(ip)), ("EBUSY from non-snapshot"));
-		vref(vp);	/* Protect against ffs_snapgone() */
+		/*
+		 * Wait for our inode block to become available.
+		 *
+		 * Hold a reference to the vnode to protect against
+		 * ffs_snapgone(). Since we hold a reference, it can only
+		 * get reclaimed (VI_DOOMED flag) in a forcible downgrade
+		 * or unmount. For an unmount, the entire filesystem will be
+		 * gone, so we cannot attempt to touch anything associated
+		 * with it while the vnode is unlocked; all we can do is 
+		 * pause briefly and try again. If when we relock the vnode
+		 * we discover that it has been reclaimed, updating it is no
+		 * longer necessary and we can just return an error.
+		 */
+		vref(vp);
 		VOP_UNLOCK(vp, 0);
-		(void) bread(ip->i_devvp,
-		     fsbtodb(fs, ino_to_fsba(fs, ip->i_number)),
-		     (int) fs->fs_bsize, NOCRED, &bp);
+		pause("ffsupd", 1);
 		vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
 		vrele(vp);
+		if ((vp->v_iflag & VI_DOOMED) != 0)
+			return (ENOENT);
+		goto loop;
 	}
 	if (DOINGSOFTDEP(vp))
 		softdep_update_inodeblock(ip, bp, waitfor);