nilfs2: fix deadlock issue between chcp and thaw ioctls

nilfs2: fix deadlock issue between chcp and thaw ioctls

An fs-thaw ioctl causes deadlock with a chcp or mkcp -s command:

 chcp            D ffff88013870f3d0     0  1325   1324 0x00000004
 ...
 Call Trace:
  [<ffffffffa06cd409>] ? nilfs_transaction_begin+0x11c/0x1a0 [nilfs2]
  [<ffffffff81056011>] ? wake_up_bit+0x20/0x20
  [<ffffffffa06d3e04>] ? copy_from_user+0x18/0x30 [nilfs2]
  [<ffffffffa06d3e99>] ? nilfs_ioctl_change_cpmode+0x7d/0xcf [nilfs2]
  [<ffffffffa06d4cf8>] ? nilfs_ioctl+0x252/0x61a [nilfs2]
  [<ffffffff813b8966>] ? do_page_fault+0x311/0x34c
  [<ffffffff810e1c2f>] ? get_unmapped_area+0x132/0x14e
  [<ffffffff8110e95c>] ? do_vfs_ioctl+0x44b/0x490
  [<ffffffff81049b7f>] ? __set_task_blocked+0x5a/0x61
  [<ffffffff810d63c1>] ? vm_mmap_pgoff+0x76/0x87
  [<ffffffff81049bb6>] ? __set_current_blocked+0x30/0x4a
  [<ffffffff8110e9ec>] ? sys_ioctl+0x4b/0x6f
  [<ffffffff813bbc79>] ? system_call_fastpath+0x16/0x1b
 thaw            D ffff88013870d890     0  1352   1351 0x00000004
 ...
 Call Trace:
  [<ffffffff813b5551>] ? rwsem_down_failed_common+0xdb/0x10f
  [<ffffffff812172c3>] ? call_rwsem_down_write_failed+0x13/0x20
  [<ffffffff813b42f4>] ? down_write+0x25/0x27
  [<ffffffff811032e7>] ? thaw_super+0x13/0x9e
  [<ffffffff8110e706>] ? do_vfs_ioctl+0x1f5/0x490
  [<ffffffff810d63c1>] ? vm_mmap_pgoff+0x76/0x87
  [<ffffffff8110e9ec>] ? sys_ioctl+0x4b/0x6f
  [<ffffffff810ff731>] ? filp_close+0x64/0x6c
  [<ffffffff813bbc79>] ? system_call_fastpath+0x16/0x1b

where the thaw ioctl deadlocked at thaw_super() when called while chcp was
waiting at nilfs_transaction_begin() called from
nilfs_ioctl_change_cpmode().  This deadlock is 100% reproducible.

This is because nilfs_ioctl_change_cpmode() first locks sb->s_umount in
read mode and then waits for unfreezing in nilfs_transaction_begin(),
whereas thaw_super() locks sb->s_umount in write mode.  The locking of
sb->s_umount here was intended to make snapshot mounts and the downgrade
of snapshots to checkpoints exclusive.

This fixes the deadlock issue by replacing the sb->s_umount usage in
nilfs_ioctl_change_cpmode() with a dedicated mutex which protects snapshot
mounts.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Tested-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>