linux-ext4 - Re: [PATCH -v2] ext4: remove usage of Uptodate flag to initialize buddy after an online resize

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Date:	Wed, 22 Oct 2008 09:56:45 +0200
From:	Frédéric Bohé <frederic.bohe@...l.net>
To:	Theodore Tso <tytso@....edu>
Cc:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH -v2] ext4: remove usage of Uptodate flag to initialize
	buddy after an online resize

Le mardi 21 octobre 2008 à 11:52 -0400, Theodore Tso a écrit :
> Hi Frederic,
> 
> Thanks for posting the update to your patch; I take it you've solved
> the race condition?  I haven't take a look at your updated patch yet,
> but one thought that might make the potential race conditions much
> simpler to analyze and prevent.

Yes the race condition I found is solved with this patch. The issue
happened when concurrent threads try to write to blocks in groups which
had been added by the resizing. As I briefly explained in the patch, it
was a matter of mballocator's datas which were wrongly initialized
several times.

> At the moment, the resize code, just before it calls to fix up the
> mballoc data structures, calls ext4_free_blocks_sb() to mark the block
> bitmap as being freed.  That call should really go away, as
> ext4_free_blocs_sb() is a remnant from the legacy block allocator, and
> in fact does a lot of extra stuff that is not needed by mballoc().
> Perhaps the right answer is that we should have one function that
> updates the block bitmap, as well as initializing the mballoc() data
> structures, and it would *only* be called from the resize code.  If

OK, I will take a look at this function and see if I can update/clean
it.

> the concern is protecting against multiple resizers running at the
> same time, then let's either (a) not call unlock_super() until the
> mballoc data structures are initialized, or (b) create a new mutex
> that is explicit for use by the online resize code.
> 

In fact, I have never tested with multiple resizers til now because I
never managed to run several instance of resize2fs concurrently: if a
resize2fs is running, the second one simply fails with a "device busy"
error.

Frederic



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