[PATCH] autofs4: fix another race between mount and expire

Jeff Moyer has identified a race between mount and expire.

What happens is that during an expire the situation can arise that a directory
is removed and another lookup is done before the expire issues a completion
status to the kernel module.  In this case, since the the lookup gets a new
dentry, it doesn't know that there is an expire in progress and when it posts
its mount request, matches the existing expire request and waits for its
completion.  ENOENT is then returned to user space from lookup (as the dentry
passed in is now unhashed) without having performed the mount request.

The solution used here is to keep track of dentrys in this unhashed state and
reuse them, if possible, in order to preserve the flags.  Additionally, this
infrastructure will provide the framework for the reintroduction of caching of
mount fails removed earlier in development.
Signed-off-by: Ian Kent <raven@themaw.net>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/autofs_i.h

@@ -52,6 +52,8 @@ struct autofs_info {
 
 	int		flags;
 
+	struct list_head rehash;
+
 	struct autofs_sb_info *sbi;
 	unsigned long last_used;
 	atomic_t count;
@@ -110,6 +112,8 @@ struct autofs_sb_info {
 	struct mutex wq_mutex;
 	spinlock_t fs_lock;
 	struct autofs_wait_queue *queues; /* Wait queue pointer */
+	spinlock_t rehash_lock;
+	struct list_head rehash_list;
 };
 
 static inline struct autofs_sb_info *autofs4_sbi(struct super_block *sb)

fs/autofs4/inode.c

@@ -48,6 +48,8 @@ struct autofs_info *autofs4_init_ino(struct autofs_info *ino,
 	ino->dentry = NULL;
 	ino->size = 0;
 
+	INIT_LIST_HEAD(&ino->rehash);
+
 	ino->last_used = jiffies;
 	atomic_set(&ino->count, 0);
 
@@ -158,14 +160,13 @@ void autofs4_kill_sb(struct super_block *sb)
 	if (!sbi)
 		goto out_kill_sb;
 
-	sb->s_fs_info = NULL;
-
-	if ( !sbi->catatonic )
+	if (!sbi->catatonic)
 		autofs4_catatonic_mode(sbi); /* Free wait queues, close pipe */
 
 	/* Clean up and release dangling references */
 	autofs4_force_release(sbi);
 
+	sb->s_fs_info = NULL;
 	kfree(sbi);
 
 out_kill_sb:
@@ -336,6 +337,8 @@ int autofs4_fill_super(struct super_block *s, void *data, int silent)
 	mutex_init(&sbi->wq_mutex);
 	spin_lock_init(&sbi->fs_lock);
 	sbi->queues = NULL;
+	spin_lock_init(&sbi->rehash_lock);
+	INIT_LIST_HEAD(&sbi->rehash_list);
 	s->s_blocksize = 1024;
 	s->s_blocksize_bits = 10;
 	s->s_magic = AUTOFS_SUPER_MAGIC;

fs/autofs4/root.c

@@ -263,7 +263,7 @@ static int try_to_fill_dentry(struct dentry *dentry, int flags)
 		 */
 		status = d_invalidate(dentry);
 		if (status != -EBUSY)
-			return -ENOENT;
+			return -EAGAIN;
 	}
 
 	DPRINTK("dentry=%p %.*s ino=%p",
@@ -413,7 +413,16 @@ static int autofs4_revalidate(struct dentry *dentry, struct nameidata *nd)
 		 */
 		status = try_to_fill_dentry(dentry, flags);
 		if (status == 0)
-				return 1;
+			return 1;
+
+		/*
+		 * A status of EAGAIN here means that the dentry has gone
+		 * away while waiting for an expire to complete. If we are
+		 * racing with expire lookup will wait for it so this must
+		 * be a revalidate and we need to send it to lookup.
+		 */
+		if (status == -EAGAIN)
+			return 0;
 
 		return status;
 	}
@@ -459,9 +468,18 @@ void autofs4_dentry_release(struct dentry *de)
 	de->d_fsdata = NULL;
 
 	if (inf) {
+		struct autofs_sb_info *sbi = autofs4_sbi(de->d_sb);
+
 		inf->dentry = NULL;
 		inf->inode = NULL;
 
+		if (sbi) {
+			spin_lock(&sbi->rehash_lock);
+			if (!list_empty(&inf->rehash))
+				list_del(&inf->rehash);
+			spin_unlock(&sbi->rehash_lock);
+		}
+
 		autofs4_free_ino(inf);
 	}
 }
@@ -478,10 +496,80 @@ static struct dentry_operations autofs4_dentry_operations = {
 	.d_release	= autofs4_dentry_release,
 };
 
+static struct dentry *autofs4_lookup_unhashed(struct autofs_sb_info *sbi, struct dentry *parent, struct qstr *name)
+{
+	unsigned int len = name->len;
+	unsigned int hash = name->hash;
+	const unsigned char *str = name->name;
+	struct list_head *p, *head;
+
+	spin_lock(&dcache_lock);
+	spin_lock(&sbi->rehash_lock);
+	head = &sbi->rehash_list;
+	list_for_each(p, head) {
+		struct autofs_info *ino;
+		struct dentry *dentry;
+		struct qstr *qstr;
+
+		ino = list_entry(p, struct autofs_info, rehash);
+		dentry = ino->dentry;
+
+		spin_lock(&dentry->d_lock);
+
+		/* Bad luck, we've already been dentry_iput */
+		if (!dentry->d_inode)
+			goto next;
+
+		qstr = &dentry->d_name;
+
+		if (dentry->d_name.hash != hash)
+			goto next;
+		if (dentry->d_parent != parent)
+			goto next;
+
+		if (qstr->len != len)
+			goto next;
+		if (memcmp(qstr->name, str, len))
+			goto next;
+
+		if (d_unhashed(dentry)) {
+			struct autofs_info *ino = autofs4_dentry_ino(dentry);
+			struct inode *inode = dentry->d_inode;
+
+			list_del_init(&ino->rehash);
+			dget(dentry);
+			/*
+			 * Make the rehashed dentry negative so the VFS
+			 * behaves as it should.
+			 */
+			if (inode) {
+				dentry->d_inode = NULL;
+				list_del_init(&dentry->d_alias);
+				spin_unlock(&dentry->d_lock);
+				spin_unlock(&sbi->rehash_lock);
+				spin_unlock(&dcache_lock);
+				iput(inode);
+				return dentry;
+			}
+			spin_unlock(&dentry->d_lock);
+			spin_unlock(&sbi->rehash_lock);
+			spin_unlock(&dcache_lock);
+			return dentry;
+		}
+next:
+		spin_unlock(&dentry->d_lock);
+	}
+	spin_unlock(&sbi->rehash_lock);
+	spin_unlock(&dcache_lock);
+
+	return NULL;
+}
+
 /* Lookups in the root directory */
 static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, struct nameidata *nd)
 {
 	struct autofs_sb_info *sbi;
+	struct dentry *unhashed;
 	int oz_mode;
 
 	DPRINTK("name = %.*s",
@@ -497,25 +585,46 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
 	DPRINTK("pid = %u, pgrp = %u, catatonic = %d, oz_mode = %d",
 		 current->pid, process_group(current), sbi->catatonic, oz_mode);
 
-	/*
-	 * Mark the dentry incomplete, but add it. This is needed so
-	 * that the VFS layer knows about the dentry, and we can count
-	 * on catching any lookups through the revalidate.
-	 *
-	 * Let all the hard work be done by the revalidate function that
-	 * needs to be able to do this anyway..
-	 *
-	 * We need to do this before we release the directory semaphore.
-	 */
-	dentry->d_op = &autofs4_root_dentry_operations;
+	unhashed = autofs4_lookup_unhashed(sbi, dentry->d_parent, &dentry->d_name);
+	if (!unhashed) {
+		/*
+		 * Mark the dentry incomplete, but add it. This is needed so
+		 * that the VFS layer knows about the dentry, and we can count
+		 * on catching any lookups through the revalidate.
+		 *
+		 * Let all the hard work be done by the revalidate function that
+		 * needs to be able to do this anyway..
+		 *
+		 * We need to do this before we release the directory semaphore.
+		 */
+		dentry->d_op = &autofs4_root_dentry_operations;
+
+		dentry->d_fsdata = NULL;
+		d_add(dentry, NULL);
+	} else {
+		struct autofs_info *ino = autofs4_dentry_ino(unhashed);
+		DPRINTK("rehash %p with %p", dentry, unhashed);
+		/*
+		 * If we are racing with expire the request might not
+		 * be quite complete but the directory has been removed
+		 * so it must have been successful, so just wait for it.
+		 */
+		if (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
+			DPRINTK("wait for incomplete expire %p name=%.*s",
+				unhashed, unhashed->d_name.len,
+				unhashed->d_name.name);
+			autofs4_wait(sbi, unhashed, NFY_NONE);
+			DPRINTK("request completed");
+		}
+		d_rehash(unhashed);
+		dentry = unhashed;
+	}
 
 	if (!oz_mode) {
 		spin_lock(&dentry->d_lock);
 		dentry->d_flags |= DCACHE_AUTOFS_PENDING;
 		spin_unlock(&dentry->d_lock);
 	}
-	dentry->d_fsdata = NULL;
-	d_add(dentry, NULL);
 
 	if (dentry->d_op && dentry->d_op->d_revalidate) {
 		mutex_unlock(&dir->i_mutex);
@@ -534,6 +643,8 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
 			if (sigismember (sigset, SIGKILL) ||
 			    sigismember (sigset, SIGQUIT) ||
 			    sigismember (sigset, SIGINT)) {
+			    if (unhashed)
+				dput(unhashed);
 			    return ERR_PTR(-ERESTARTNOINTR);
 			}
 		}
@@ -548,8 +659,14 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
 	 * doesn't do the right thing for all system calls, but it should
 	 * be OK for the operations we permit from an autofs.
 	 */
-	if (dentry->d_inode && d_unhashed(dentry))
+	if (dentry->d_inode && d_unhashed(dentry)) {
+		if (unhashed)
+			dput(unhashed);
 		return ERR_PTR(-ENOENT);
+	}
+
+	if (unhashed)
+		return dentry;
 
 	return NULL;
 }
@@ -611,9 +728,10 @@ static int autofs4_dir_symlink(struct inode *dir,
  * Normal filesystems would do a "d_delete()" to tell the VFS dcache
  * that the file no longer exists. However, doing that means that the
  * VFS layer can turn the dentry into a negative dentry.  We don't want
- * this, because since the unlink is probably the result of an expire.
- * We simply d_drop it, which allows the dentry lookup to remount it
- * if necessary.
+ * this, because the unlink is probably the result of an expire.
+ * We simply d_drop it and add it to a rehash candidates list in the
+ * super block, which allows the dentry lookup to reuse it retaining
+ * the flags, such as expire in progress, in case we're racing with expire.
  *
  * If a process is blocked on the dentry waiting for the expire to finish,
  * it will invalidate the dentry and try to mount with a new one.
@@ -642,7 +760,14 @@ static int autofs4_dir_unlink(struct inode *dir, struct dentry *dentry)
 
 	dir->i_mtime = CURRENT_TIME;
 
-	d_drop(dentry);
+	spin_lock(&dcache_lock);
+	spin_lock(&sbi->rehash_lock);
+	list_add(&ino->rehash, &sbi->rehash_list);
+	spin_unlock(&sbi->rehash_lock);
+	spin_lock(&dentry->d_lock);
+	__d_drop(dentry);
+	spin_unlock(&dentry->d_lock);
+	spin_unlock(&dcache_lock);
 
 	return 0;
 }
@@ -653,6 +778,9 @@ static int autofs4_dir_rmdir(struct inode *dir, struct dentry *dentry)
 	struct autofs_info *ino = autofs4_dentry_ino(dentry);
 	struct autofs_info *p_ino;
 	
+	DPRINTK("dentry %p, removing %.*s",
+		dentry, dentry->d_name.len, dentry->d_name.name);
+
 	if (!autofs4_oz_mode(sbi))
 		return -EACCES;
 
@@ -661,6 +789,9 @@ static int autofs4_dir_rmdir(struct inode *dir, struct dentry *dentry)
 		spin_unlock(&dcache_lock);
 		return -ENOTEMPTY;
 	}
+	spin_lock(&sbi->rehash_lock);
+	list_add(&ino->rehash, &sbi->rehash_list);
+	spin_unlock(&sbi->rehash_lock);
 	spin_lock(&dentry->d_lock);
 	__d_drop(dentry);
 	spin_unlock(&dentry->d_lock);