linux-kernel - Re: 2.6.34 echo j > /proc/sysrq-trigger causes inifnite unfreeze/Thaw event

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Message-ID: <4C0C7448.7040409@sandeen.net>
Date:	Sun, 06 Jun 2010 23:23:36 -0500
From:	Eric Sandeen <sandeen@...deen.net>
To:	Dave Chinner <david@...morbit.com>
CC:	Jeffrey Merkey <jeffmerkey@...il.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	josef@...hat.com, viro@...iv.linux.org.uk
Subject: Re: 2.6.34 echo j > /proc/sysrq-trigger causes inifnite unfreeze/Thaw
 event

Dave Chinner wrote:
> On Thu, Jun 03, 2010 at 11:30:30PM -0600, Jeffrey Merkey wrote:
>> causes the FS Thaw stuff in fs/buffer.c to enter an infinite loop
>> filling the /var/log/messages with junk and causing the hard drive to
>> crank away endlessly.
> 
> Hmmm, looks pretty obvious what the 2.6.34 bug is:
> 
>         while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
>                 printk(KERN_WARNING "Emergency Thaw on %s\n",
>                        bdevname(sb->s_bdev, b));
> 
> thaw_bdev() returns 0 on success or not frozen, and returns non-zero
> only if the unfreeze failed. Looks like it was broken from the start
> to me.

thaw_bdev() used to return -EINVAL for not-frozen, but it no longer
does.  Seems commit 4504230a dropped this:

-       if (!bdev->bd_fsfreeze_count) {
-               mutex_unlock(&bdev->bd_fsfreeze_mutex);
-               return -EINVAL;
-       }

in favor of this:

+       int error = -EINVAL;
...
+       if (!bdev->bd_fsfreeze_count)
+               goto out_unlock;
...
+out_unlock:
        mutex_unlock(&bdev->bd_fsfreeze_mutex);
        return 0;

and now we return 0 for a thaw of a non-frozen bdev ....
this breaks the loop in the emergency thaw-er (which was
just supposed to work its way through nested freezes,
when I wrote it)

-Eric
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