linux-kernel - Re: [PATCH -v2] f2fs: Remove lock from check_valid

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Message-ID: <20140908035024.GA13009@jaegeuk-mac02.hsd1.ca.comcast.net>
Date:	Sun, 7 Sep 2014 20:50:24 -0700
From:	Jaegeuk Kim <jaegeuk@...nel.org>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Changman Lee <cm224.lee@...sung.com>,
	linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -v2] f2fs: Remove lock from check_valid_map

Hi,

On Sun, Sep 07, 2014 at 11:38:30AM +0800, Huang Ying wrote:
> Only one bit is read in check_valid_map, holding a lock to do that
> doesn't help anything except decreasing performance.
> 
> Signed-off-by: Huang, Ying <ying.huang@...el.com>
> ---
> 
> v2: Fixed a build warning.
> 
> ---
>  fs/f2fs/gc.c |    3 ---
>  1 file changed, 3 deletions(-)
> 
> --- a/fs/f2fs/gc.c
> +++ b/fs/f2fs/gc.c
> @@ -378,14 +378,11 @@ static void put_gc_inode(struct list_hea
>  static int check_valid_map(struct f2fs_sb_info *sbi,
>  				unsigned int segno, int offset)
>  {
> -	struct sit_info *sit_i = SIT_I(sbi);
>  	struct seg_entry *sentry;
>  	int ret;
>  
> -	mutex_lock(&sit_i->sentry_lock);
>  	sentry = get_seg_entry(sbi, segno);
>  	ret = f2fs_test_bit(offset, sentry->cur_valid_map);
> -	mutex_unlock(&sit_i->sentry_lock);
>  	return ret;

The f2fs_test_bit is not atomic, so I'm not sure this is a good approach.
How about introducing rw_semaphore?

>  }
>  
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