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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Sun, 29 Sep 2013 18:04:57 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r255941 - head/sys/kern
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Author: kib
Date: Sun Sep 29 18:04:57 2013
New Revision: 255941
URL: http://svnweb.freebsd.org/changeset/base/255941

Log:
  Reimplement r255797 using LK_TRYUPGRADE.
  
  The  r255797 was:
  Increase the chance of the buffer write from the bufdaemon helper
  context to succeed.  If the locked vnode which owns the buffer to be
  written is shared locked, try the non-blocking upgrade of the lock to
  exclusive.
  
  PR:	kern/178997
  Reported and tested by:	Klaus Weber <fbsd-bugs-2013-1@unix-admin.de>
  Sponsored by:	The FreeBSD Foundation
  MFC after:	1 week
  Approved by:	re (glebius)

Modified:
  head/sys/kern/vfs_bio.c

Modified: head/sys/kern/vfs_bio.c
==============================================================================
--- head/sys/kern/vfs_bio.c	Sun Sep 29 18:02:23 2013	(r255940)
+++ head/sys/kern/vfs_bio.c	Sun Sep 29 18:04:57 2013	(r255941)
@@ -2624,6 +2624,8 @@ flushbufqueues(struct vnode *lvp, int ta
 	int hasdeps;
 	int flushed;
 	int queue;
+	int error;
+	bool unlock;
 
 	flushed = 0;
 	queue = QUEUE_DIRTY;
@@ -2699,7 +2701,16 @@ flushbufqueues(struct vnode *lvp, int ta
 			BUF_UNLOCK(bp);
 			continue;
 		}
-		if (vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT | LK_CANRECURSE) == 0) {
+		if (lvp == NULL) {
+			unlock = true;
+			error = vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT);
+		} else {
+			ASSERT_VOP_LOCKED(vp, "getbuf");
+			unlock = false;
+			error = VOP_ISLOCKED(vp) == LK_EXCLUSIVE ? 0 :
+			    vn_lock(vp, LK_TRYUPGRADE);
+		}
+		if (error == 0) {
 			mtx_unlock(&bqdirty);
 			CTR3(KTR_BUF, "flushbufqueue(%p) vp %p flags %X",
 			    bp, bp->b_vp, bp->b_flags);
@@ -2711,7 +2722,8 @@ flushbufqueues(struct vnode *lvp, int ta
 				notbufdflushes++;
 			}
 			vn_finished_write(mp);
-			VOP_UNLOCK(vp, 0);
+			if (unlock)
+				VOP_UNLOCK(vp, 0);
 			flushwithdeps += hasdeps;
 			flushed++;