Merge tag 'pull-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

Pull /proc/sys dcache lookup fix from Al Viro:
 "Fix for the breakage spotted by Neil in the interplay between
  /proc/sys ->d_compare() weirdness and parallel lookups"

* tag 'pull-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  fix proc_sys_compare() handling of in-lookup dentries

Author: torvalds

fs/proc/inode.c

@@ -42,7 +42,7 @@ static void proc_evict_inode(struct inode *inode)
 
 	head = ei->sysctl;
 	if (head) {
-		RCU_INIT_POINTER(ei->sysctl, NULL);
+		WRITE_ONCE(ei->sysctl, NULL);
 		proc_sys_evict_inode(inode, head);
 	}
 }

fs/proc/proc_sysctl.c

@@ -918,17 +918,21 @@ static int proc_sys_compare(const struct dentry *dentry,
 	struct ctl_table_header *head;
 	struct inode *inode;
 
-	/* Although proc doesn't have negative dentries, rcu-walk means
-	 * that inode here can be NULL */
-	/* AV: can it, indeed? */
-	inode = d_inode_rcu(dentry);
-	if (!inode)
-		return 1;
 	if (name->len != len)
 		return 1;
 	if (memcmp(name->name, str, len))
 		return 1;
-	head = rcu_dereference(PROC_I(inode)->sysctl);
+
+	// false positive is fine here - we'll recheck anyway
+	if (d_in_lookup(dentry))
+		return 0;
+
+	inode = d_inode_rcu(dentry);
+	// we just might have run into dentry in the middle of __dentry_kill()
+	if (!inode)
+		return 1;
+
+	head = READ_ONCE(PROC_I(inode)->sysctl);
 	return !head || !sysctl_is_seen(head);
 }