tree 76c0185a960d9c7fd17f68d3f5ba61b4e0533f3d
parent 6282f4f8bd912cfab3f112f1b3e3f3386521fa06
author Amir Goldstein <amir@cellrox.com> 1429862075 +0300
committer Mattias Nissler <mnissler@google.com> 1469024298 +0200

FROMLIST: sysrq: Emergency Remount R/O in reverse order

This change fixes a problem where reboot on Android panics the kernel
almost every time when file systems are mounted over loop devices.

Android reboot command does:
- sync
- echo u > /proc/sysrq-trigger
- syscall_reboot

The problem is with sysrq emergency remount R/O trying to remount-ro
in wrong order.
since /data is re-mounted ro before loop devices, loop device
remount-ro fails to flush the journal and panics the kernel:

  EXT4-fs (loop0): Remounting filesystem read-only
  EXT4-fs (loop0): previous I/O error to superblock detected
  loop: Write error at byte offset 0, length 4096.
  Buffer I/O error on device loop0, logical block 0
  lost page write due to I/O error on loop0
  Kernel panic - not syncing: EXT4-fs panic from previous error

The fix is quite simple. In do_emergency_remount(), use
list_for_each_entry_reverse() on sb list instead of list_for_each_entry().
It makes a lot of sense to umount the file systems in reverse order in
which they were added to sb list.

Signed-off-by: Amir Goldstein <amir@cellrox.com>
Acked-by: Oren Laadan <orenl@cellrox.com>

Bug: None
Patchset: sysrq-remount-reverse

(from https://patchwork.ozlabs.org/patch/464146/)
Signed-off-by: Mattias Nissler <mnissler@google.com>

Change-Id: Ifadab7bfef0b6bcfc1e8509cde968bd77c2dc705