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Date:      Tue, 4 Aug 2020 23:04:30 +0000 (UTC)
From:      Mateusz Guzik <mjg@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r363872 - in head/sys: kern sys
Message-ID:  <202008042304.074N4UN0026724@repo.freebsd.org>
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Author: mjg
Date: Tue Aug  4 23:04:29 2020
New Revision: 363872
URL: https://svnweb.freebsd.org/changeset/base/363872

Log:
  cache: add cache_purge_vgone
  
  cache_purge locklessly checks whether the vnode at hand has any namecache
  entries. This can race with a concurrent purge which managed to remove
  the last entry, but may not be done touching the vnode.
  
  Make sure we observe the relevant vnode lock as not taken before proceeding
  with vgone.
  
  Paired with the fact that doomed vnodes cannnot receive entries this restores
  the invariant that there are no namecache-related writing users past cache_purge
  in vgone.
  
  Reported by:	pho

Modified:
  head/sys/kern/vfs_cache.c
  head/sys/kern/vfs_subr.c
  head/sys/sys/vnode.h

Modified: head/sys/kern/vfs_cache.c
==============================================================================
--- head/sys/kern/vfs_cache.c	Tue Aug  4 23:00:00 2020	(r363871)
+++ head/sys/kern/vfs_cache.c	Tue Aug  4 23:04:29 2020	(r363872)
@@ -2138,22 +2138,17 @@ cache_changesize(u_long newmaxvnodes)
 /*
  * Invalidate all entries from and to a particular vnode.
  */
-void
-cache_purge(struct vnode *vp)
+static void
+cache_purge_impl(struct vnode *vp)
 {
 	TAILQ_HEAD(, namecache) ncps;
 	struct namecache *ncp, *nnp;
 	struct mtx *vlp, *vlp2;
 
-	CTR1(KTR_VFS, "cache_purge(%p)", vp);
-	SDT_PROBE1(vfs, namecache, purge, done, vp);
-	if (LIST_EMPTY(&vp->v_cache_src) && TAILQ_EMPTY(&vp->v_cache_dst) &&
-	    vp->v_cache_dd == NULL)
-		return;
 	TAILQ_INIT(&ncps);
 	vlp = VP2VNODELOCK(vp);
 	vlp2 = NULL;
-	mtx_lock(vlp);
+	mtx_assert(vlp, MA_OWNED);
 retry:
 	while (!LIST_EMPTY(&vp->v_cache_src)) {
 		ncp = LIST_FIRST(&vp->v_cache_src);
@@ -2182,6 +2177,53 @@ retry:
 	TAILQ_FOREACH_SAFE(ncp, &ncps, nc_dst, nnp) {
 		cache_free(ncp);
 	}
+}
+
+void
+cache_purge(struct vnode *vp)
+{
+	struct mtx *vlp;
+
+	SDT_PROBE1(vfs, namecache, purge, done, vp);
+	if (LIST_EMPTY(&vp->v_cache_src) && TAILQ_EMPTY(&vp->v_cache_dst) &&
+	    vp->v_cache_dd == NULL)
+		return;
+	vlp = VP2VNODELOCK(vp);
+	mtx_lock(vlp);
+	cache_purge_impl(vp);
+}
+
+/*
+ * Only to be used by vgone.
+ */
+void
+cache_purge_vgone(struct vnode *vp)
+{
+	struct mtx *vlp;
+
+	VNPASS(VN_IS_DOOMED(vp), vp);
+	vlp = VP2VNODELOCK(vp);
+	if (!(LIST_EMPTY(&vp->v_cache_src) && TAILQ_EMPTY(&vp->v_cache_dst) &&
+	    vp->v_cache_dd == NULL)) {
+		mtx_lock(vlp);
+		cache_purge_impl(vp);
+		mtx_assert(vlp, MA_NOTOWNED);
+		return;
+	}
+
+	/*
+	 * All the NULL pointer state we found above may be transient.
+	 * Serialize against a possible thread doing cache_purge.
+	 */
+	mtx_wait_unlocked(vlp);
+	if (!(LIST_EMPTY(&vp->v_cache_src) && TAILQ_EMPTY(&vp->v_cache_dst) &&
+	    vp->v_cache_dd == NULL)) {
+		mtx_lock(vlp);
+		cache_purge_impl(vp);
+		mtx_assert(vlp, MA_NOTOWNED);
+		return;
+	}
+	return;
 }
 
 /*

Modified: head/sys/kern/vfs_subr.c
==============================================================================
--- head/sys/kern/vfs_subr.c	Tue Aug  4 23:00:00 2020	(r363871)
+++ head/sys/kern/vfs_subr.c	Tue Aug  4 23:04:29 2020	(r363872)
@@ -4146,7 +4146,7 @@ vgonel(struct vnode *vp)
 	 * Delete from old mount point vnode list.
 	 */
 	delmntque(vp);
-	cache_purge(vp);
+	cache_purge_vgone(vp);
 	/*
 	 * Done with purge, reset to the standard lock and invalidate
 	 * the vnode.

Modified: head/sys/sys/vnode.h
==============================================================================
--- head/sys/sys/vnode.h	Tue Aug  4 23:00:00 2020	(r363871)
+++ head/sys/sys/vnode.h	Tue Aug  4 23:04:29 2020	(r363872)
@@ -638,6 +638,7 @@ int	cache_lookup(struct vnode *dvp, struct vnode **vpp
 	    struct componentname *cnp, struct timespec *tsp, int *ticksp);
 void	cache_vnode_init(struct vnode *vp);
 void	cache_purge(struct vnode *vp);
+void	cache_purge_vgone(struct vnode *vp);
 void	cache_purge_negative(struct vnode *vp);
 void	cache_purgevfs(struct mount *mp, bool force);
 int	change_dir(struct vnode *vp, struct thread *td);

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