[rfc][patch 1/2] inotify: fix race

[rfc][patch 1/2] inotify: fix race

[Nick Piggin]

Hmm, thinking I'd better just take a hammer to this inotify problem
despite the fact I cannot confirm the root cause of the warning messages.
Possibilities include the race being fixed here, and races in the
debug code itself.

--

There is a race between setting an inode's children's "parent watched" flag
when placing the first watch on a parent, and instantiating new children of
that parent: a child could miss having its flags set by set_dentry_child_flags,
but then inotify_d_instantiate might still see !inotify_inode_watched.

The solution is to set_dentry_child_flags after adding the watch. Locking is
taken care of, because both set_dentry_child_flags and inotify_d_instantiate
hold dcache_lock and child->d_locks.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/fs/inotify.c
===================================================================
--- linux-2.6.orig/fs/inotify.c
+++ linux-2.6/fs/inotify.c
@@ -627,6 +627,7 @@ s32 inotify_add_watch(struct inotify_han
 		      struct inode *inode, u32 mask)
 {
 	int ret = 0;
+	int newly_watched;
 
 	/* don't allow invalid bits: we don't want flags set */
 	mask &= IN_ALL_EVENTS | IN_ONESHOT;
@@ -653,12 +654,18 @@ s32 inotify_add_watch(struct inotify_han
 	 */
 	watch->inode = igrab(inode);
 
-	if (!inotify_inode_watched(inode))
-		set_dentry_child_flags(inode, 1);
-
 	/* Add the watch to the handle's and the inode's list */
+	newly_watched = !inotify_inode_watched(inode);
 	list_add(&watch->h_list, &ih->watches);
 	list_add(&watch->i_list, &inode->inotify_watches);
+	/*
+	 * Set child flags _after_ adding the watch, so there is no race
+	 * windows where newly instantiated children could miss their parent's
+	 * watched flag.
+	 */
+	if (newly_watched)
+		set_dentry_child_flags(inode, 1);
+
 out:
 	mutex_unlock(&ih->mutex);
 	mutex_unlock(&inode->inotify_mutex);

[rfc][patch 2/2] inotify: remove debug code

[Nick Piggin]

The inotify debugging code is supposed to verify that the
DCACHE_INOTIFY_PARENT_WATCHED scalability optimisation does not result in
notifications getting lost nor extra needless locking generated.

Unfortunately there are also some races in the debugging code. And it isn't
very good at finding problems anyway. So remove it for now.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c
+++ linux-2.6/fs/dcache.c
@@ -1408,9 +1408,6 @@ void d_delete(struct dentry * dentry)
 	if (atomic_read(&dentry->d_count) == 1) {
 		dentry_iput(dentry);
 		fsnotify_nameremove(dentry, isdir);
-
-		/* remove this and other inotify debug checks after 2.6.18 */
-		dentry->d_flags &= ~DCACHE_INOTIFY_PARENT_WATCHED;
 		return;
 	}
 
Index: linux-2.6/fs/inotify.c
===================================================================
--- linux-2.6.orig/fs/inotify.c
+++ linux-2.6/fs/inotify.c
@@ -168,20 +168,14 @@ static void set_dentry_child_flags(struc
 		struct dentry *child;
 
 		list_for_each_entry(child, &alias->d_subdirs, d_u.d_child) {
-			if (!child->d_inode) {
-				WARN_ON(child->d_flags & DCACHE_INOTIFY_PARENT_WATCHED);
+			if (!child->d_inode)
 				continue;
-			}
+
 			spin_lock(&child->d_lock);
-			if (watched) {
-				WARN_ON(child->d_flags &
-						DCACHE_INOTIFY_PARENT_WATCHED);
+			if (watched)
 				child->d_flags |= DCACHE_INOTIFY_PARENT_WATCHED;
-			} else {
-				WARN_ON(!(child->d_flags &
-					DCACHE_INOTIFY_PARENT_WATCHED));
-				child->d_flags&=~DCACHE_INOTIFY_PARENT_WATCHED;
-			}
+			else
+				child->d_flags &=~DCACHE_INOTIFY_PARENT_WATCHED;
 			spin_unlock(&child->d_lock);
 		}
 	}
@@ -253,7 +247,6 @@ void inotify_d_instantiate(struct dentry
 	if (!inode)
 		return;
 
-	WARN_ON(entry->d_flags & DCACHE_INOTIFY_PARENT_WATCHED);
 	spin_lock(&entry->d_lock);
 	parent = entry->d_parent;
 	if (parent->d_inode && inotify_inode_watched(parent->d_inode))