Re: [RFC PATCH] writeback: move list_lock down into the for loop

[Michal Hocko]

On Fri 26-02-16 08:46:25, Yang Shi wrote:
> The list_lock was moved outside the for loop by commit
> e8dfc30582995ae12454cda517b17d6294175b07 ("writeback: elevate queue_io()
> into wb_writeback())", however, the commit log says "No behavior change", so
> it sounds safe to have the list_lock acquired inside the for loop as it did
> before.
> Leave tracepoints outside the critical area since tracepoints already have
> preempt disabled.

The patch says what but it completely misses the why part.

> 
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>
> ---
> Tested with ltp on 8 cores Cortex-A57 machine.
> 
>  fs/fs-writeback.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 1f76d89..9b7b5f6 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1623,7 +1623,6 @@ static long wb_writeback(struct bdi_writeback *wb,
>  	work->older_than_this = &oldest_jif;
>  
>  	blk_start_plug(&plug);
> -	spin_lock(&wb->list_lock);
>  	for (;;) {
>  		/*
>  		 * Stop writeback when nr_pages has been consumed
> @@ -1661,15 +1660,19 @@ static long wb_writeback(struct bdi_writeback *wb,
>  			oldest_jif = jiffies;
>  
>  		trace_writeback_start(wb, work);
> +
> +		spin_lock(&wb->list_lock);
>  		if (list_empty(&wb->b_io))
>  			queue_io(wb, work);
>  		if (work->sb)
>  			progress = writeback_sb_inodes(work->sb, wb, work);
>  		else
>  			progress = __writeback_inodes_wb(wb, work);
> -		trace_writeback_written(wb, work);
>  
>  		wb_update_bandwidth(wb, wb_start);
> +		spin_unlock(&wb->list_lock);
> +
> +		trace_writeback_written(wb, work);
>  
>  		/*
>  		 * Did we write something? Try for more
> @@ -1693,15 +1696,14 @@ static long wb_writeback(struct bdi_writeback *wb,
>  		 */
>  		if (!list_empty(&wb->b_more_io))  {
>  			trace_writeback_wait(wb, work);
> +			spin_lock(&wb->list_lock);
>  			inode = wb_inode(wb->b_more_io.prev);
> -			spin_lock(&inode->i_lock);
>  			spin_unlock(&wb->list_lock);
> +			spin_lock(&inode->i_lock);
>  			/* This function drops i_lock... */
>  			inode_sleep_on_writeback(inode);
> -			spin_lock(&wb->list_lock);
>  		}
>  	}
> -	spin_unlock(&wb->list_lock);
>  	blk_finish_plug(&plug);
>  
>  	return nr_pages - work->nr_pages;
> -- 
> 2.0.2
> 
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-- 
Michal Hocko
SUSE Labs