linux-ext4 - Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

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Date:	Thu, 5 May 2011 00:48:24 +0200
From:	Jan Kara <jack@...e.cz>
To:	Surbhi Palande <surbhi.palande@...onical.com>
Cc:	Jan Kara <jack@...e.cz>,
	Toshiyuki Okajima <toshi.okajima@...fujitsu.com>,
	Ted Ts'o <tytso@....edu>,
	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due
 to a deadlock

On Thu 05-05-11 00:34:51, Surbhi Palande wrote:
> On 05/04/2011 10:19 PM, Jan Kara wrote:
> >On Wed 04-05-11 15:09:37, Surbhi Palande wrote:
> >>On 05/03/2011 06:19 PM, Jan Kara wrote:
> >>>On Tue 03-05-11 14:01:50, Surbhi Palande wrote:
> >>>>What happens in the case as follows:
> >>>>
> >>>>Task 1: Mmapped writes
> >>>>t1)ext4_page_mkwrite()
> >>>>   t2) ext4_write_begin() (FS is thawed so we proceed)
> >>>>   t3) ext4_write_end() (journal is stopped now)
> >>>>-----Pre-empted-----
> >>>>
> >>>>
> >>>>Task 2: Freeze Task
> >>>>t4) freezes the super block...
> >>>>...(continues)....
> >>>>tn) the page cache is clean and the F.S is frozen. Freeze has
> >>>>completed execution.
> >>>>
> >>>>Task 1: Mmapped writes
> >>>>tn+1) ext4_page_mkwrite() returns 0.
> >>>>tn+2) __do_fault() gets control, code gets executed.
> >>>>tn+3) _do_fault() marks the page dirty if the intent is to write to
> >>>>a file based page which faulted.
> >>>>
> >>>>So you end up dirtying the page cache when the F.S is frozen? No?
> >>>   You are right ext4_page_mkrite() as currently implemented has problems.
> >>>You have to return the page locked (and check for frozen fs with page lock
> >>>held) to avoid races.
> >>>
> >>>If you check for frozen fs with page lock held, you are guaranteed that
> >>>freezing code must wait for the page to get unlocked before proceeding. And
> >>>before the page is unlocked, it is marked dirty by the pagefault code which
> >>>makes freezing code write the page and writeprotect it again. So everything
> >>>will be safe.
> >>For the locked page to be a part of the freeze initiated sync,
> >>should its owner inode not be dirtied? The page fault handler
> >>dirties the page, but who ensures that the inode is dirtied at this
> >>point?
> Well, I mean it as follows:
> 
> Doesn't the writeback code (invoked via sync_filesystem(sb)) write
> all the dirty pages of all the _dirty_ inodes of a superblock?
> 
> So in the window from the point where ext4_page_mkwrite returns to
> __do_fault() _till_ you mark the inode dirty (in
> __mark_inode_dirty()), you can have a race with freeze i.e if freeze
> happens meanwhile, then the sync initiated by freeze will not
> consider this locked page as the owner inode is _clean_ (or not
> dirtied yet) at that point?
  Ah, I see. That's actually a good point! Thanks for persistence. So we
should also dirty the page before checking for frozen fs.

> Key: tx: time at unit x
> 
> P1: mmapped writes
> t1) __do_page_fault()
>    t2) ext4_page_mkwrite()
>       // owner inode of the page is in _clean_ state - not yet dirtied
>    --- pre-empted---
> 
> P2: Freeze_super
> tn) freeze_super gets control
> freezes the F.S, skips the owner inode as it is in the clean state.
> syncs all the other dirty inodes. page cache is now clean.
> 
> 
> P1: mmapped writes (resume)
> tn+x)__do_page_fault() gets control back:
>    tn+x+1) set_page_dirty()
>      tn+x+2) __set_page_dirty_buffers()
>         tn+x+3) __set_page_dirty()
>  	   tn+x+4) radix_tree_tag_set(page, PAGECACHE_TAG_DIRTY)
> 
> So don't we end up dirtying the page cache when the F.S is frozen?
> 
> Again, apologies if I understood the writeback code or something else wrong!
  No, you understood it right. Just your previous email was too generic so
I have not thought about this particular race.

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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