svn commit: r324804 - head/sys/kern

[Mark Johnston]

Author: markj
Date: Fri Oct 20 14:56:13 2017
New Revision: 324804
URL: https://svnweb.freebsd.org/changeset/base/324804

Log:
  Avoid the nbp lookup in the final loop iteration in flushbuflist().
  
  The end of the loop must re-lookup the next buf since the bufobj lock
  is dropped in the loop body. If the lookup fails, the loop is restarted.
  This mechanism non-obviously also terminates the loop when the end of
  the buf list is reached. Split up the two loops termination cases to
  make the code a bit less fragile. No functional change intended.
  
  Reviewed by:	kib
  MFC after:	1 week
  Sponsored by:	Dell EMC Isilon
  Differential Revision:	https://reviews.freebsd.org/D12730

Modified:
  head/sys/kern/vfs_subr.c

Modified: head/sys/kern/vfs_subr.c
==============================================================================
--- head/sys/kern/vfs_subr.c	Fri Oct 20 14:04:01 2017	(r324803)
+++ head/sys/kern/vfs_subr.c	Fri Oct 20 14:56:13 2017	(r324804)
@@ -1746,8 +1746,6 @@ flushbuflist(struct bufv *bufv, int flags, struct bufo
 		    ((flags & V_ALT) && (bp->b_xflags & BX_ALTDATA) == 0)) {
 			continue;
 		}
-		lblkno = 0;
-		xflags = 0;
 		if (nbp != NULL) {
 			lblkno = nbp->b_lblkno;
 			xflags = nbp->b_xflags & (BX_VNDIRTY | BX_VNCLEAN);
@@ -1782,6 +1780,8 @@ flushbuflist(struct bufv *bufv, int flags, struct bufo
 		bp->b_flags &= ~B_ASYNC;
 		brelse(bp);
 		BO_LOCK(bo);
+		if (nbp == NULL)
+			break;
 		nbp = gbincore(bo, lblkno);
 		if (nbp == NULL || (nbp->b_xflags & (BX_VNDIRTY | BX_VNCLEAN))
 		    != xflags)