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From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Tue, 15 Nov 2016 03:38:05 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r308665 - head/sys/kern
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Author: mjg
Date: Tue Nov 15 03:38:05 2016
New Revision: 308665
URL: https://svnweb.freebsd.org/changeset/base/308665

Log:
  cache: fix a race between entry removal and demotion
  
  The negative list shrinker can demote an entry with only hotlist + neglist
  locks held. On the other hand entry removal possibly sets the NCF_DVDROP
  without aformentioned locks held prior to detaching it from the respective
  netlist., which can lose the update made by the shrinker.
  
  Reported and tested by:	truckman

Modified:
  head/sys/kern/vfs_cache.c

Modified: head/sys/kern/vfs_cache.c
==============================================================================
--- head/sys/kern/vfs_cache.c	Tue Nov 15 02:36:12 2016	(r308664)
+++ head/sys/kern/vfs_cache.c	Tue Nov 15 03:38:05 2016	(r308665)
@@ -868,6 +868,13 @@ cache_zap_locked(struct namecache *ncp, 
 		    nc_get_name(ncp), ncp->nc_neghits);
 	}
 	LIST_REMOVE(ncp, nc_hash);
+	if (!(ncp->nc_flag & NCF_NEGATIVE)) {
+		TAILQ_REMOVE(&ncp->nc_vp->v_cache_dst, ncp, nc_dst);
+		if (ncp == ncp->nc_vp->v_cache_dd)
+			ncp->nc_vp->v_cache_dd = NULL;
+	} else {
+		cache_negative_remove(ncp, neg_locked);
+	}
 	if (ncp->nc_flag & NCF_ISDOTDOT) {
 		if (ncp == ncp->nc_dvp->v_cache_dd)
 			ncp->nc_dvp->v_cache_dd = NULL;
@@ -878,13 +885,6 @@ cache_zap_locked(struct namecache *ncp, 
 			atomic_subtract_rel_long(&numcachehv, 1);
 		}
 	}
-	if (!(ncp->nc_flag & NCF_NEGATIVE)) {
-		TAILQ_REMOVE(&ncp->nc_vp->v_cache_dst, ncp, nc_dst);
-		if (ncp == ncp->nc_vp->v_cache_dd)
-			ncp->nc_vp->v_cache_dd = NULL;
-	} else {
-		cache_negative_remove(ncp, neg_locked);
-	}
 	atomic_subtract_rel_long(&numcache, 1);
 }