On 05/02/2011 02:27 PM, Surbhi Palande wrote:
On 05/02/2011 01:56 PM, Jan Kara wrote:
On Mon 02-05-11 12:07:59, Surbhi Palande wrote:
On 04/06/2011 02:21 PM, Dave Chinner wrote:
On Wed, Apr 06, 2011 at 08:18:56AM +0200, Jan Kara wrote:
On Wed 06-04-11 15:40:05, Dave Chinner wrote:
On Fri, Apr 01, 2011 at 04:08:56PM +0200, Jan Kara wrote:
On Fri 01-04-11 10:40:50, Dave Chinner wrote:
If you don't allow the page to be dirtied in the fist place, then
nothing needs to be done to the writeback path because there is
nothing dirty for it to write back.
Sure but that's only the problem he was able to hit. But generally,
there's a problem with needing s_umount for unfreezing because it
isn't
clear there aren't other code paths which can block with s_umount
held
waiting for fs to get unfrozen. And these code paths would cause
the same
deadlock. That's why I chose to get rid of s_umount during thawing.
Holding the s_umount lock while checking if frozen and sleeping
is essentially an ABBA lock inversion bug that can bite in many more
places that just thawing the filesystem. Any where this is done
should
be fixed, so I don't think just removing the s_umount lock from
the thaw
path is sufficient to avoid problems.
That's easily said but hard to do - any transaction start in ext3/4
may
block on filesystem being frozen (this seems to be similar for XFS
as I'm
looking into the code) and transaction start traditionally nests
inside
s_umount (and basically there's no way around that since sync()
calls your
fs code with s_umount held).
Sure, but the question must be asked - why is ext3/4 even starting a
transaction on a clean filesystem during sync? A frozen filesystem,
by definition, is a clean filesytem, and therefore sync calls of any
kind should not be trying to write to the FS or start transactions.
XFS does this just fine, so I'd consider such behaviour on a frozen
filesystem a bug in ext3/4...
I had a look at the xfs code for seeing how this is done.
xfs_file_aio_write()
xfs_wait_for_freeze()
vfs_check_frozen()
So xfs_file_aio_write() writes to buffers when the FS is not frozen.
Now, I want to know what stops the following scenario from happening:
--------------------
xfs_file_aio_write()
xfs_wait_for_freeze()
vfs_check_frozen()
At this point F.S was not frozen, so the next instruction in the
xfs_file_aio_write() will be executed next.
However at this point (i.e after checking if F.S is frozen) the
write process gets pre-empted and say the _freeze_ process gets
control.
Now the F.S freezes and the write process gets the control back. And
so we end up writing to the page cache when the F.S is frozen.
--------------------
Can anyone please enlighten me on how& why this premption is _not_
possible?
Thanks for your reply.
XFS works similarly as ext4 in this regard I believe. They have the log
frozen in xfs_freeze() so if the race you describe above happens, either
the writing process gets caught waiting for log to unfreeze
Agreed.
or it manages
to start a transaction and then freezing process waits for transaction to
finish before it can proceed with freezing. I'm not sure why is there the
check in xfs_file_aio_write()...
I am sorry, but I don't understand how this will happen - i.e I can't
understand what stops freeze_super() (or ext4_freeze) from freezing a
superblock (as the write process stopped just before writing anything
for this transaction and has not taken any locks?)
To make myself a little more coherent:
freeze_super()
ext4_freeze()
1) jbd2_journal_updates()
2) jbd2_journal_flush(journal)
3) jbd2_journal_unlock_updates(journal).
4) return
Say now the fs write process stopped just after checking that fs is not
frozen (i.e its thawed). So its ready to write to the page cache. Just
when it has finished this vfs_check_frozen() and before it starts any
write (or transactions), say the write process gets pre-empted and then
the freeze process freezes the superblock. Wont ext4_freeze() simply
lock the current transactions, flush them to the log and then unlock the
transactions (so that new handles/transactions can be accepted later?)
So then after the fsfreeze finishes freezing the F.S, say if the write
process gets the control back. The write process assumes that after its
out of vfs_check_frozen(), the fs is thawed (or unfrozen) where as in
this case it is not.
So I don't understand, _what_ stops the writing process from starting a
transaction in this case when the F.S is frozen already
and what stops the fsfreeze from waiting for the write process (when it
has not yet started the write)?
Warm Regards,
Surbhi.
Thanks!
Warm Regards,
Surbhi.
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