linux-kernel - Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish

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Date:	Fri, 11 Sep 2015 13:37:19 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Chris Mason <clm@...com>, LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Josef Bacik <jbacik@...com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

On Fri, Sep 11, 2015 at 1:02 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> How about we instead:
>
>  (a) revert that commit d353d7587 as broken (because it clearly is)
>
>  (b) add a big honking comment about the fact that we hold 'list_lock'
> in writeback_sb_inodes()
>
>  (c) move the plugging up to wb_writeback() and writeback_inodes_wb()
> _outside_ the spinlock.

Ok, I've done (a) and (b) in my tree. And attached is the totally
untested patch for (c). It looks ObviouslyCorrect(tm), but since this
is a performance issue, I'm not going to commit it without some more
ACK's from people.

I obviously think this is a *much* better approach than dropping and
retaking the lock, but there might be something silly I'm missing.

For example, maybe we want to unplug and replug around the
"inode_sleep_on_writeback()" in wb_writeback()? So while the revert
was a no-brainer, this one I really want people to think about.

                       Linus

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