RE: [PATCH 2/3] ocfs2: Remove a useless spinlock

From: David Laight <David.Laight@ACULAB.COM>
To: 'Christophe JAILLET' <christophe.jaillet@wanadoo.fr>,
	Mark Fasheh <mark@fasheh.com>, Joel Becker <jlbec@evilplan.org>,
	Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kernel-janitors@vger.kernel.org"
	<kernel-janitors@vger.kernel.org>,
	"ocfs2-devel@oss.oracle.com" <ocfs2-devel@oss.oracle.com>
Subject: RE: [PATCH 2/3] ocfs2: Remove a useless spinlock
Date: Tue, 19 Jul 2022 10:24:41 +0000	[thread overview]
Message-ID: <7b644e5d32d74d3d90dfc5b1786ae5b9@AcuMS.aculab.com> (raw)
In-Reply-To: <8ba7004d330cbe5f626539a8a3bff696d0c4285e.1658224839.git.christophe.jaillet@wanadoo.fr>

From: Christophe JAILLET
> Sent: 19 July 2022 11:02
> 
> 'node_map_lock' is a spinlock only used to protect calls to set_bit(),
> clear_bit() and test_bit().
> 
> {set|clear}_bit() are already atomic and don't need this extra spinlock.
> test_bit() only reads the bitmap for a given bit.
> 
> Remove this useless spinlock.

It looks to me like the calling code is racy
unless there is another lock in the callers.
While map->map is protected, the result of test_bit()
is stale - so can't be used for much.

	David

> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
> test_bit() is NOT documented as an atomic function. However, I can't see
> how it could return a wrong result here.
> 
> So review with care. There is maybe something I don't think about that is
> lurking here.
> ---
>  fs/ocfs2/heartbeat.c | 11 ++++-------
>  fs/ocfs2/ocfs2.h     |  2 --
>  2 files changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/ocfs2/heartbeat.c b/fs/ocfs2/heartbeat.c
> index 1d72e0788943..4863ad35c242 100644
> --- a/fs/ocfs2/heartbeat.c
> +++ b/fs/ocfs2/heartbeat.c
> @@ -35,7 +35,6 @@ static void ocfs2_node_map_init(struct ocfs2_node_map *map)
> 
>  void ocfs2_init_node_maps(struct ocfs2_super *osb)
>  {
> -	spin_lock_init(&osb->node_map_lock);
>  	ocfs2_node_map_init(&osb->osb_recovering_orphan_dirs);
>  }
> 
> @@ -67,9 +66,8 @@ void ocfs2_node_map_set_bit(struct ocfs2_super *osb,
>  	if (bit==-1)
>  		return;
>  	BUG_ON(bit >= map->num_nodes);
> -	spin_lock(&osb->node_map_lock);
> +
>  	set_bit(bit, map->map);
> -	spin_unlock(&osb->node_map_lock);
>  }
> 
>  void ocfs2_node_map_clear_bit(struct ocfs2_super *osb,
> @@ -79,9 +77,8 @@ void ocfs2_node_map_clear_bit(struct ocfs2_super *osb,
>  	if (bit==-1)
>  		return;
>  	BUG_ON(bit >= map->num_nodes);
> -	spin_lock(&osb->node_map_lock);
> +
>  	clear_bit(bit, map->map);
> -	spin_unlock(&osb->node_map_lock);
>  }
> 
>  int ocfs2_node_map_test_bit(struct ocfs2_super *osb,
> @@ -89,13 +86,13 @@ int ocfs2_node_map_test_bit(struct ocfs2_super *osb,
>  			    int bit)
>  {
>  	int ret;
> +
>  	if (bit >= map->num_nodes) {
>  		mlog(ML_ERROR, "bit=%d map->num_nodes=%d\n", bit, map->num_nodes);
>  		BUG();
>  	}
> -	spin_lock(&osb->node_map_lock);
> +
>  	ret = test_bit(bit, map->map);
> -	spin_unlock(&osb->node_map_lock);
>  	return ret;
>  }
> 
> diff --git a/fs/ocfs2/ocfs2.h b/fs/ocfs2/ocfs2.h
> index 740b64238312..1df193b97c30 100644
> --- a/fs/ocfs2/ocfs2.h
> +++ b/fs/ocfs2/ocfs2.h
> @@ -302,8 +302,6 @@ struct ocfs2_super
> 
>  	u32 *slot_recovery_generations;
> 
> -	spinlock_t node_map_lock;
> -
>  	u64 root_blkno;
>  	u64 system_dir_blkno;
>  	u64 bitmap_blkno;
> --
> 2.34.1

-
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