jbd: check for error returned by kthread_create on creating journal thread

If the thread failed to create the subsequent wait_event will hang forever.

This is likely to happen if kernel hits max_threads limit.

Will be critical for virtualization systems that limit the number of tasks
and kernel memory usage within the container.

(akpm: JBD should be converted fully to the kthread API: kthread_should_stop()
and kthread_stop()).

Cc: <linux-ext4@vger.kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/jbd/journal.c

@@ -210,10 +210,16 @@ end_loop:
 	return 0;
 }
 
-static void journal_start_thread(journal_t *journal)
+static int journal_start_thread(journal_t *journal)
 {
-	kthread_run(kjournald, journal, "kjournald");
+	struct task_struct *t;
+
+	t = kthread_run(kjournald, journal, "kjournald");
+	if (IS_ERR(t))
+		return PTR_ERR(t);
+
 	wait_event(journal->j_wait_done_commit, journal->j_task != 0);
+	return 0;
 }
 
 static void journal_kill_thread(journal_t *journal)
@@ -839,8 +845,7 @@ static int journal_reset(journal_t *journal)
 
 	/* Add the dynamic fields and write it to disk. */
 	journal_update_superblock(journal, 1);
-	journal_start_thread(journal);
-	return 0;
+	return journal_start_thread(journal);
 }
 
 /**

fs/jbd2/journal.c

@@ -210,10 +210,16 @@ end_loop:
 	return 0;
 }
 
-static void jbd2_journal_start_thread(journal_t *journal)
+static int jbd2_journal_start_thread(journal_t *journal)
 {
-	kthread_run(kjournald2, journal, "kjournald2");
+	struct task_struct *t;
+
+	t = kthread_run(kjournald2, journal, "kjournald2");
+	if (IS_ERR(t))
+		return PTR_ERR(t);
+
 	wait_event(journal->j_wait_done_commit, journal->j_task != 0);
+	return 0;
 }
 
 static void journal_kill_thread(journal_t *journal)
@@ -839,8 +845,7 @@ static int journal_reset(journal_t *journal)
 
 	/* Add the dynamic fields and write it to disk. */
 	jbd2_journal_update_superblock(journal, 1);
-	jbd2_journal_start_thread(journal);
-	return 0;
+	return jbd2_journal_start_thread(journal);
 }
 
 /**