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SubjectRe: 2.6.34 echo j > /proc/sysrq-trigger causes inifnite unfreeze/Thaw event
On Mon, Jun 07, 2010 at 11:05:42AM +1000, 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.
> Fixing that endless loop shows some other problems on 2.6.35,
> though: the emergency unfreeze is not unfreezing frozen XFS
> filesystems. This appears to be caused by
> 18e9e5104fcd9a973ffe3eed3816c87f2a1b6cd2 ("Introduce freeze_super
> and thaw_super for the fsfreeze ioctl").
> It appears that this introduces a significant mismatch between the
> bdev freeze/thaw and the super freze/thaw. That is, if you freeze
> with the sb method, you can only unfreeze via the sb method.
> however, if you freeze via the bdev method, you can unfreeze by
> either the bdev or sb method. This breaks the nesting of the
> freeze/thaw operations between dm and userspace, which can lead to
> premature thawing of the filesystem.
> Then there is this deadlock:
> iterate_supers(do_thaw_one) does:
> down_read(&sb->s_umount);
> do_thaw_one(sb)
> thaw_bdev(sb->s_bdev, sb))
> thaw_super(sb)
> down_write(&sb->s_umount);
> Which is an instant deadlock.
> These problems were hidden by the fact that the emergency thaw code
> was not getting past the thaw_bdev guards and so not triggering
> this deadlock.
> Al, Josef, what's the best way to fix this mess?

Well we can do something like the following

1) Make a __thaw_super() that just does all the work currently in thaw_super(),
just without taking the s_umount semaphore.
2) Make an thaw_bdev_force or something like that that just sets
bd_fsfreeze_count to 0 and calls __thaw_super(). The original intent was to
make us call thaw until the thaw actually occured, so might as well just make it
quick and painless.
3) Make do_thaw_one() call __thaw_super if sb->s_bdev doesn't exist. I'm not
sure if this happens currently, but it's nice just in case.

This takes care of the s_umount problem and makes sure that do_thaw_one does
actually thaw the device. Does this sound kosher to everybody? Thanks,


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