On 3/31/11 3:37 AM, Yongqiang Yang wrote:
> in ext3, ext3_freeze() prevents journal from being updated by
> lock_journal_updates(), ext3_unfreeze() allow journal to be updated by
> unlock_journal_updates().
>
> in ext4, however, before ext4_freeze() returns, it unlock journal, and
> ext4 prevents journal from being updated by s_frozen. s_frozen is in
> an upper layer, so it is out control of ext4 and deadlock is easy to
> happen.
>
> Could someone explain why ext4 does like above but not follow ext3?
>
> Yongqiang.
That was me, I think ...
commit 6b0310fbf087ad6e9e3b8392adca97cd77184084
Author: Eric Sandeen <sandeen@xxxxxxxxxx>
Date: Sun May 16 02:00:00 2010 -0400
ext4: don't return to userspace after freezing the fs with a mutex held
ext4_freeze() used jbd2_journal_lock_updates() which takes
the j_barrier mutex, and then returns to userspace. The
kernel does not like this:
================================================
[ BUG: lock held when returning to user space! ]
------------------------------------------------
lvcreate/1075 is leaving the kernel with locks still held!
1 lock held by lvcreate/1075:
#0: (&journal->j_barrier){+.+...}, at: [<ffffffff811c6214>]
jbd2_journal_lock_updates+0xe1/0xf0
Use vfs_check_frozen() added to ext4_journal_start_sb() and
ext4_force_commit() instead.
Addresses-Red-Hat-Bugzilla: #568503
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
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