linux-kernel - Re: [PATCH 2/9] writeback: switch to per-bdi threads for flushing data

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Message-ID: <20090806214433.GA866@infradead.org>
Date:	Thu, 6 Aug 2009 17:44:33 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, chris.mason@...cle.com,
	david@...morbit.com, hch@...radead.org, akpm@...ux-foundation.org,
	yanmin_zhang@...ux.intel.com, richard@....demon.co.uk,
	damien.wyart@...e.fr, fweisbec@...il.com, Alan.Brunelle@...com
Subject: Re: [PATCH 2/9] writeback: switch to per-bdi threads for flushing
	data

On Wed, Aug 05, 2009 at 06:35:32PM +0200, Jan Kara wrote:
>   Jens, sorry about bitching about this again but you're silently changing
> substantial locking assumptions without writing it *anywhere* and without
> arguing it's safe.
>   Originally, generic_sync_sb_inodes() from writeback path have been
> called with
> a) s_umount_sem held
> b) sb->s_count elevated
>   The second still seems to be true since, if I'm right, we pass here
> non-NULL sb only from sync_filesystem() and that takes care of the
> superblock reference. So that is just a matter of documenting this fact
> before the function.

We'll defintively need to keep both to prevent races vs unmount.  And
with a NULL superblock passed I'm not even sure how we can take care of
it.

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