autofs4: deal with autofs4_write/autofs4_write races

Just serialize the actual writing of packets into pipe on
a new mutex, independent from everything else in the locking
hierarchy.  As soon as something has started feeding a piece
of packet into the pipe to daemon, we *want* everything else
about to try the same to wait until we are done.

Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/autofs4/autofs_i.h

@@ -116,6 +116,7 @@ struct autofs_sb_info {
 	int needs_reghost;
 	struct super_block *sb;
 	struct mutex wq_mutex;
+	struct mutex pipe_mutex;
 	spinlock_t fs_lock;
 	struct autofs_wait_queue *queues; /* Wait queue pointer */
 	spinlock_t lookup_lock;

fs/autofs4/inode.c

@@ -225,6 +225,7 @@ int autofs4_fill_super(struct super_block *s, void *data, int silent)
 	sbi->min_proto = 0;
 	sbi->max_proto = 0;
 	mutex_init(&sbi->wq_mutex);
+	mutex_init(&sbi->pipe_mutex);
 	spin_lock_init(&sbi->fs_lock);
 	sbi->queues = NULL;
 	spin_lock_init(&sbi->lookup_lock);

fs/autofs4/waitq.c

@@ -56,26 +56,27 @@ void autofs4_catatonic_mode(struct autofs_sb_info *sbi)
 	mutex_unlock(&sbi->wq_mutex);
 }
 
-static int autofs4_write(struct file *file, const void *addr, int bytes)
+static int autofs4_write(struct autofs_sb_info *sbi,
+			 struct file *file, const void *addr, int bytes)
 {
 	unsigned long sigpipe, flags;
 	mm_segment_t fs;
 	const char *data = (const char *)addr;
 	ssize_t wr = 0;
 
-	/** WARNING: this is not safe for writing more than PIPE_BUF bytes! **/
-
 	sigpipe = sigismember(&current->pending.signal, SIGPIPE);
 
 	/* Save pointer to user space and point back to kernel space */
 	fs = get_fs();
 	set_fs(KERNEL_DS);
 
+	mutex_lock(&sbi->pipe_mutex);
 	while (bytes &&
 	       (wr = file->f_op->write(file,data,bytes,&file->f_pos)) > 0) {
 		data += wr;
 		bytes -= wr;
 	}
+	mutex_lock(&sbi->pipe_mutex);
 
 	set_fs(fs);
 
@@ -179,7 +180,7 @@ static void autofs4_notify_daemon(struct autofs_sb_info *sbi,
 
 	mutex_unlock(&sbi->wq_mutex);
 
-	if (autofs4_write(pipe, &pkt, pktsz))
+	if (autofs4_write(sbi, pipe, &pkt, pktsz))
 		autofs4_catatonic_mode(sbi);
 	fput(pipe);
 }