linux-ext4 - Re: [BUG] invalid superblock checksum possibly due to race

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Date:   Wed, 1 Jul 2020 18:22:48 +0200
From:   Jan Kara <jack@...e.cz>
To:     Costa Sapuntzakis <costa@...estorage.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [BUG] invalid superblock checksum possibly due to race

Hello!

On Tue 30-06-20 11:34:49, Costa Sapuntzakis wrote:
> > Yes, probably ext4_superblock_csum_set() should use
> >
> > lock_buffer(EXT4_SB(sb)->s_sbh)
> >
> > to synchronize updating of superblock checksum. Will you send a patch?
> 
> Yes. I will send a patch.

Thanks!

> I noticed lock_buffer can sleep. That would seem to imply to me that
> lock_buffer can be held across I/Os.
> I worry that this will occasionally significantly slow down this code
> path compared to what it used to be.  Are there any things
> about the way ext4 uses buffers that makes this less of a concern?

Yes, buffer lock is a sleeping lock but that's the lock we usually use to
protect consistency of buffer contents. So I prefer to use that lock unless
we have definitive performance data showing we need something more
clever...

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