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
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