linux-ext4 - Re: [PATCH] ext4: block direct I/O writes during ext4

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Message-ID: <20130716164942.GA6002@thunk.org>
Date:	Tue, 16 Jul 2013 12:49:42 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH] ext4: block direct I/O writes during ext4_truncate

On Tue, Jul 16, 2013 at 05:46:58PM +0200, Jan Kara wrote:
> and ext4_setattr() does (again under i_mutex):
>                                         ext4_inode_block_unlocked_dio(inode);
>                                         inode_dio_wait(inode);
>                                         ext4_inode_resume_unlocked_dio(inode);

Ah, I missed this; thanks for pointing this out.

> So either DIO gets i_mutex first and then ext4_setattr() waits for it to
> complete, or truncate completes before unlocked DIO is able to get & drop
> i_mutex.
> 
> OTOH unlocked DIO *read* might be vulnerable to a race with truncate. That
> never acquires i_mutex so if the DIO read arrives after ext4_setattr() goes
> through inode_dio_wait(), we can have the read and truncate racing and read
> possibly submitting read of a just truncated block (which can get
> reallocated in theory while the IO is running).
> 
> So something like what you do in the patch is likely needed, just the
> justification is somewhat different and you should also rip out / adjust
> the other synchronizations we have in ext4_setattr(), ext4_ext_direct_IO()
> and ext4_ind_direct_IO().

Ok, I'll drop my current patch for now, and revisit this when I have a
bit more time.  I agree with your analysis, but fortunately it sounds
like this race is going to be pretty hard to hit in practice ---
especially, since with the journal enabled, we won't allow the block
to get reused until the next commit boundary.  The situation where we
would need to worry would be dioread_nolock combined with no journal
mode.

						- Ted
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