linux-kernel - Re: [PATCH v2 3/8] document rwsem_release() in sb_wait

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Date:	Thu, 13 Aug 2015 12:22:19 +0200
From:	Jan Kara <jack@...e.cz>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jan Kara <jack@...e.cz>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/8] document rwsem_release() in sb_wait_write()

On Tue 11-08-15 19:04:04, Oleg Nesterov wrote:
> Not only we need to avoid the warning from lockdep_sys_exit(), the
> caller of freeze_super() can never release this lock. Another thread
> can do this, so there is another reason for rwsem_release().
> 
> Plus the comment should explain why we have to fool lockdep.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Looks good. You can add:

Reviewed-by: Jan Kara <jack@...e.com>

								Honza

> ---
>  fs/super.c |   12 +++++++++---
>  1 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index d0fdd49..89b58fb 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1236,11 +1236,17 @@ static void sb_wait_write(struct super_block *sb, int level)
>  {
>  	s64 writers;
>  
> +	rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
>  	/*
> -	 * We just cycle-through lockdep here so that it does not complain
> -	 * about returning with lock to userspace
> +	 * We are going to return to userspace and forget about this lock, the
> +	 * ownership goes to the caller of thaw_super() which does unlock.
> +	 *
> +	 * FIXME: we should do this before return from freeze_super() after we
> +	 * called sync_filesystem(sb) and s_op->freeze_fs(sb), and thaw_super()
> +	 * should re-acquire these locks before s_op->unfreeze_fs(sb). However
> +	 * this leads to lockdep false-positives, so currently we do the early
> +	 * release right after acquire.
>  	 */
> -	rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
>  	rwsem_release(&sb->s_writers.lock_map[level-1], 1, _THIS_IP_);
>  
>  	do {
> -- 
> 1.5.5.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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