ext4: avoid panic during forced reboot

When admin calls "reboot -f" - i.e., does a hard system reboot by
directly calling reboot(2) - ext4 filesystem mounted with errors=panic
can panic the system. This happens because the underlying device gets
disabled without unmounting the filesystem and thus some syscall running
in parallel to reboot(2) can result in the filesystem getting IO errors.

This is somewhat surprising to the users so try improve the behavior by
switching to errors=remount-ro behavior when the system is running
reboot(2).
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -430,6 +430,12 @@ static void ext4_journal_commit_callback(journal_t *journal, transaction_t *txn)
 	spin_unlock(&sbi->s_md_lock);
 }
 
+static bool system_going_down(void)
+{
+	return system_state == SYSTEM_HALT || system_state == SYSTEM_POWER_OFF
+		|| system_state == SYSTEM_RESTART;
+}
+
 /* Deal with the reporting of failure conditions on a filesystem such as
  * inconsistencies detected or read IO failures.
  *
@@ -460,7 +466,12 @@ static void ext4_handle_error(struct super_block *sb)
 		if (journal)
 			jbd2_journal_abort(journal, -EIO);
 	}
-	if (test_opt(sb, ERRORS_RO)) {
+	/*
+	 * We force ERRORS_RO behavior when system is rebooting. Otherwise we
+	 * could panic during 'reboot -f' as the underlying device got already
+	 * disabled.
+	 */
+	if (test_opt(sb, ERRORS_RO) || system_going_down()) {
 		ext4_msg(sb, KERN_CRIT, "Remounting filesystem read-only");
 		/*
 		 * Make sure updated value of ->s_mount_flags will be visible
@@ -468,8 +479,7 @@ static void ext4_handle_error(struct super_block *sb)
 		 */
 		smp_wmb();
 		sb->s_flags |= SB_RDONLY;
-	}
-	if (test_opt(sb, ERRORS_PANIC)) {
+	} else if (test_opt(sb, ERRORS_PANIC)) {
 		if (EXT4_SB(sb)->s_journal &&
 		  !(EXT4_SB(sb)->s_journal->j_flags & JBD2_REC_ERR))
 			return;