On Fri, May 13, 2011 at 07:04:32AM +0800, Dave Chinner wrote:
> On Thu, May 12, 2011 at 09:57:19PM +0800, Wu Fengguang wrote:
> > When wbc.more_io was first introduced, it indicates whether there are
> > at least one superblock whose s_more_io contains more IO work. Now with
> > the per-bdi writeback, it can be replaced with a simple b_more_io test.
> >
> > Acked-by: Jan Kara <jack@xxxxxxx>
> > Acked-by: Mel Gorman <mel@xxxxxxxxx>
> > CC: Minchan Kim <minchan.kim@xxxxxxxxx>
> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > ---
> > fs/fs-writeback.c | 9 ++-------
> > include/linux/writeback.h | 1 -
> > include/trace/events/ext4.h | 6 ++----
> > include/trace/events/writeback.h | 5 +----
> > 4 files changed, 5 insertions(+), 16 deletions(-)
> >
> > --- linux-next.orig/fs/fs-writeback.c 2011-05-05 23:30:30.000000000 +0800
> > +++ linux-next/fs/fs-writeback.c 2011-05-05 23:30:33.000000000 +0800
> > @@ -560,12 +560,8 @@ static int writeback_sb_inodes(struct su
> > iput(inode);
> > cond_resched();
> > spin_lock(&wb->list_lock);
> > - if (wbc->nr_to_write <= 0) {
> > - wbc->more_io = 1;
> > + if (wbc->nr_to_write <= 0)
> > return 1;
> > - }
> > - if (!list_empty(&wb->b_more_io))
> > - wbc->more_io = 1;
> > }
> > /* b_io is empty */
> > return 1;
> > @@ -707,7 +703,6 @@ static long wb_writeback(struct bdi_writ
> > wbc.older_than_this = &oldest_jif;
> > }
> >
> > - wbc.more_io = 0;
> > wbc.nr_to_write = write_chunk;
> > wbc.pages_skipped = 0;
> > wbc.inodes_cleaned = 0;
> > @@ -755,7 +750,7 @@ retry:
> > /*
> > * No more inodes for IO, bail
> > */
> > - if (!wbc.more_io)
> > + if (list_empty(&wb->b_more_io))
> > break;
>
> We're not holding the wb->list_lock here, so we need to be careful
> here. I think this is safe given that there shuold only be one
> flusher thread operating on the list, but when we expand to multiple
> flusher threads per-bdi, this coul dbe a nasty landmine. A comment
> is probably in order explaining why this is safe to check unlocked
> right now...
OK, how about this?
/*
* No more inodes for IO, bail. The unlocked check is safe
* because each &wb will be worked by only one flusher thread.
*/
if (list_empty(&wb->b_more_io))
break;
I guess in future multiple flusher threads will be working on
different bdi_writeback instances, so it will still be safe.
However for now there are possible interactions from the IO-full
balance_dirty_pages(). So it looks better to just do the tests inside
the lock:
--- linux-next.orig/fs/fs-writeback.c 2011-05-13 13:02:18.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-05-13 13:02:52.000000000 +0800
@@ -710,6 +710,7 @@ static long wb_writeback(struct bdi_writ
oldest_jif = jiffies;
work->older_than_this = &oldest_jif;
+ spin_lock(&wb->list_lock);
for (;;) {
/*
* Stop writeback when nr_pages has been consumed
@@ -742,14 +743,12 @@ static long wb_writeback(struct bdi_writ
retry:
trace_writeback_start(wb->bdi, work);
- spin_lock(&wb->list_lock);
if (list_empty(&wb->b_io))
queue_io(wb, work->older_than_this);
if (work->sb)
progress = writeback_sb_inodes(work->sb, wb, work);
else
progress = __writeback_inodes_wb(wb, work);
- spin_unlock(&wb->list_lock);
trace_writeback_written(wb->bdi, work);
/*
@@ -785,7 +784,6 @@ retry:
* become available for writeback. Otherwise
* we'll just busyloop.
*/
- spin_lock(&wb->list_lock);
if (!list_empty(&wb->b_more_io)) {
trace_writeback_wait(wb->bdi, work);
inode = wb_inode(wb->b_more_io.prev);
@@ -793,8 +791,8 @@ retry:
inode_wait_for_writeback(inode, wb);
spin_unlock(&inode->i_lock);
}
- spin_unlock(&wb->list_lock);
}
+ spin_unlock(&wb->list_lock);
return nr_pages - work->nr_pages;
}
--
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