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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Tue, 27 Aug 2013 01:31:12 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r254945 - head/sys/kern
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Author: kib
Date: Tue Aug 27 01:31:12 2013
New Revision: 254945
URL: http://svnweb.freebsd.org/changeset/base/254945

Log:
  When allocating a pbuf for the cluster write, do not sleep waiting
  for the available pbuf when passed vnode is backing md(4). Other i/o
  directed to the same md device might already hold pbufs, and then we
  could deadlock since only our progress can free a pbuf needed for
  wakeup.
  
  Obtained from:	projects/vm6
  Reminded and tested by:	pho
  MFC after:	1 week

Modified:
  head/sys/kern/vfs_cluster.c

Modified: head/sys/kern/vfs_cluster.c
==============================================================================
--- head/sys/kern/vfs_cluster.c	Tue Aug 27 01:08:55 2013	(r254944)
+++ head/sys/kern/vfs_cluster.c	Tue Aug 27 01:31:12 2013	(r254945)
@@ -837,7 +837,9 @@ cluster_wbuild(struct vnode *vp, long si
 		  (tbp->b_bcount != tbp->b_bufsize) ||
 		  (tbp->b_bcount != size) ||
 		  (len == 1) ||
-		  ((bp = getpbuf(&cluster_pbuf_freecnt)) == NULL)) {
+		  ((bp = (vp->v_vflag & VV_MD) != 0 ?
+		  trypbuf(&cluster_pbuf_freecnt) :
+		  getpbuf(&cluster_pbuf_freecnt)) == NULL)) {
 			totalwritten += tbp->b_bufsize;
 			bawrite(tbp);
 			++start_lbn;