linux-kernel - Re: [PATCH 6/6] kill-the-BKL/reiserfs: release the write lock on flush_commit

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Message-ID: <20090501054247.GD5983@elte.hu>
Date:	Fri, 1 May 2009 07:42:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jeff Mahoney <jeffm@...e.com>,
	ReiserFS Development List <reiserfs-devel@...r.kernel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Alexander Beregalov <a.beregalov@...il.com>,
	Alessio Igor Bogani <abogani@...ware.it>,
	Jonathan Corbet <corbet@....net>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 6/6] kill-the-BKL/reiserfs: release the write lock on
	flush_commit_list()


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> flush_commit_list() uses ll_rw_block() to commit the pending log blocks.
> ll_rw_block() might sleep, and the bkl was released at this point. Then
> we can also relax the write lock at this point.
> 
> [ Impact: release the reiserfs write lock when it is not needed ]
> 
> Cc: Jeff Mahoney <jeffm@...e.com>
> Cc: Chris Mason <chris.mason@...cle.com>
> Cc: Alexander Beregalov <a.beregalov@...il.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  fs/reiserfs/journal.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/reiserfs/journal.c b/fs/reiserfs/journal.c
> index 373d080..b1ebd5a 100644
> --- a/fs/reiserfs/journal.c
> +++ b/fs/reiserfs/journal.c
> @@ -1120,8 +1120,11 @@ static int flush_commit_list(struct super_block *s,
>  		    SB_ONDISK_JOURNAL_SIZE(s);
>  		tbh = journal_find_get_block(s, bn);
>  		if (tbh) {
> -			if (buffer_dirty(tbh))
> -			    ll_rw_block(WRITE, 1, &tbh) ;
> +			if (buffer_dirty(tbh)) {
> +		            reiserfs_write_unlock(s);
> +			    ll_rw_block(WRITE, 1, &tbh);
> +			    reiserfs_write_lock(s);
> +			}
>  			put_bh(tbh) ;
>  		}
>  	}

there's 7 other instances of ll_rw_block():

fs/reiserfs/journal.c-			spin_unlock(lock);
fs/reiserfs/journal.c:			ll_rw_block(WRITE, 1, &bh);
fs/reiserfs/journal.c-			spin_lock(lock);
--
fs/reiserfs/journal.c-		            reiserfs_write_unlock(s);
fs/reiserfs/journal.c:			    ll_rw_block(WRITE, 1, &tbh);
fs/reiserfs/journal.c-			    reiserfs_write_lock(s);
--
fs/reiserfs/journal.c-	/* read in the log blocks, memcpy to the corresponding real block */
fs/reiserfs/journal.c:	ll_rw_block(READ, get_desc_trans_len(desc), log_blocks);
fs/reiserfs/journal.c-	for (i = 0; i < get_desc_trans_len(desc); i++) {
--
fs/reiserfs/journal.c-		set_buffer_dirty(real_blocks[i]);
fs/reiserfs/journal.c:		ll_rw_block(SWRITE, 1, real_blocks + i);
fs/reiserfs/journal.c-	}
--
fs/reiserfs/journal.c-	}
fs/reiserfs/journal.c:	ll_rw_block(READ, j, bhlist);
fs/reiserfs/journal.c-	for (i = 1; i < j; i++)
--
fs/reiserfs/stree.c-		if (!buffer_uptodate(bh[j]))
fs/reiserfs/stree.c:			ll_rw_block(READA, 1, bh + j);
fs/reiserfs/stree.c-		brelse(bh[j]);
--
fs/reiserfs/stree.c-						    reada_blocks, reada_count);
fs/reiserfs/stree.c:			ll_rw_block(READ, 1, &bh);
fs/reiserfs/stree.c-			reiserfs_write_unlock(sb);
--
fs/reiserfs/super.c-{
fs/reiserfs/super.c:	ll_rw_block(READ, 1, &(SB_BUFFER_WITH_SB(s)));
fs/reiserfs/super.c-	reiserfs_write_unlock(s);

in particular the second stree.c one and the super.c has a 
write-unlock straight before the lock-drop.

I think the stree.c unlock could be moved to before the 
ll_rw_block() call straight away.

The super.c one needs more care: first put &(SB_BUFFER_WITH_SB(s)) 
into a local variable, then unlock the wite-lock, then call 
ll_rw_block(). (This is important because &(SB_BUFFER_WITH_SB(s)) is 
global filesystem state that has to be read with the lock held.)

ll_rw_block() generally always has a chance to block (especially on 
READ) - so the other places could be converted to drop the 
write-lock too. Most seem straightforward - some need similar 
local-variable treatment as super.c.

	Ingo
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