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Date:      Mon, 9 Sep 2019 11:22:38 +0000 (UTC)
From:      Konstantin Belousov <kib@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r352058 - head/sys/ufs/ffs
Message-ID:  <201909091122.x89BMc0D061510@repo.freebsd.org>
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Author: kib
Date: Mon Sep  9 11:22:38 2019
New Revision: 352058
URL: https://svnweb.freebsd.org/changeset/base/352058

Log:
  Remove some unneeded vfs_busy() calls in SU code.
  
  When softdep_fsync() is running, a caller must already started write
  for the mount point.  Since unmount or remount to ro suspends mount
  point, it cannot run in parallel with softdep_fsync(), which makes
  vfs_busy() call there not needed.
  
  Doing blocking vfs_busy() there effectively causes lock order reversal
  between vn_start_write() and setting MNTK_UNMOUNT, because
  vfs_busy(mp, 0) sleeps waiting for MNTK_UNMOUNT becoming clear, while
  unmount sets the flag and starts the suspension.
  
  Note that all other uses of vfs_busy() in SU code are non-blocking.
  
  Reported by:	chs by mckusick
  Reviewed by:	mckusick
  Tested by:	pho
  Sponsored by:	The FreeBSD Foundation
  MFC after:	1 week

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

Modified: head/sys/ufs/ffs/ffs_softdep.c
==============================================================================
--- head/sys/ufs/ffs/ffs_softdep.c	Mon Sep  9 11:20:15 2019	(r352057)
+++ head/sys/ufs/ffs/ffs_softdep.c	Mon Sep  9 11:22:38 2019	(r352058)
@@ -12507,24 +12507,13 @@ restart:
 		FREE_LOCK(ump);
 		if (ffs_vgetf(mp, parentino, LK_NOWAIT | LK_EXCLUSIVE, &pvp,
 		    FFSV_FORCEINSMQ)) {
-			error = vfs_busy(mp, MBF_NOWAIT);
-			if (error != 0) {
-				vfs_ref(mp);
-				VOP_UNLOCK(vp, 0);
-				error = vfs_busy(mp, 0);
-				vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
-				vfs_rel(mp);
-				if (error != 0)
-					return (ENOENT);
-				if (vp->v_iflag & VI_DOOMED) {
-					vfs_unbusy(mp);
-					return (ENOENT);
-				}
-			}
+			/*
+			 * Unmount cannot proceed after unlock because
+			 * caller must have called vn_start_write().
+			 */
 			VOP_UNLOCK(vp, 0);
 			error = ffs_vgetf(mp, parentino, LK_EXCLUSIVE,
 			    &pvp, FFSV_FORCEINSMQ);
-			vfs_unbusy(mp);
 			vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
 			if (vp->v_iflag & VI_DOOMED) {
 				if (error == 0)

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