Re: [PATCH 2/8] writeback: move dirty inodes from super_block to backing_dev_info

[Christoph Hellwig <hch@infradead.org>]

On Sun, Sep 06, 2009 at 08:43:59PM +0200, Jens Axboe wrote:
> On Fri, Sep 04 2009, Christoph Hellwig wrote:
> > On Fri, Sep 04, 2009 at 08:53:57AM +0200, Jens Axboe wrote:
> > > > +	if (wbc->sync_mode == WB_SYNC_ALL)
> > > > +		bdi_wait_on_work_clear(&work);
> > > >  }
> > > 
> > > That doesn't work, you have to wait for on-stack work. So either we just
> > > punt and not do anything for WB_SYNC_NONE if the allocation fails, or we
> > > punt to stack and do the wait. Since it's a cleaning action and
> > > allocation fails, falling back to the stack and waiting seems like the
> > > most appropriate choice.
> > 
> > True, the wait needs to be unconditional.  Updated version below.
> 
> (did you forget that patch? it's not there).

Here we go, sorry:


Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c	2009-09-04 12:37:27.022522276 -0300
+++ linux-2.6/fs/fs-writeback.c	2009-09-04 12:37:50.618022253 -0300
@@ -70,13 +70,6 @@ static inline void bdi_work_init(struct 
 	work->state = WS_USED;
 }
 
-static inline void bdi_work_init_on_stack(struct bdi_work *work,
-					  struct writeback_control *wbc)
-{
-	bdi_work_init(work, wbc);
-	work->state |= WS_ONSTACK;
-}
-
 /**
  * writeback_in_progress - determine whether there is writeback in progress
  * @bdi: the device's backing_dev_info structure.
@@ -207,35 +200,21 @@ static struct bdi_work *bdi_alloc_work(s
 
 void bdi_start_writeback(struct writeback_control *wbc)
 {
-	const bool must_wait = wbc->sync_mode == WB_SYNC_ALL;
-	struct bdi_work work_stack, *work = NULL;
-
-	if (!must_wait)
-		work = bdi_alloc_work(wbc);
+	struct bdi_work work;
 
-	if (!work) {
-		work = &work_stack;
-		bdi_work_init_on_stack(work, wbc);
+	if (wbc->sync_mode != WB_SYNC_ALL) {
+		struct bdi_work *w = bdi_alloc_work(wbc);
+		if (w) {
+			bdi_queue_work(wbc->bdi, w);
+			return;
+		}
 	}
 
-	bdi_queue_work(wbc->bdi, work);
+	bdi_work_init(&work, wbc);
+	work.state |= WS_ONSTACK;
 
-	/*
-	 * If the sync mode is WB_SYNC_ALL, block waiting for the work to
-	 * complete. If not, we only need to wait for the work to be started,
-	 * if we allocated it on-stack. We use the same mechanism, if the
-	 * wait bit is set in the bdi_work struct, then threads will not
-	 * clear pending until after they are done.
-	 *
-	 * Note that work == &work_stack if must_wait is true, so we don't
-	 * need to do call_rcu() here ever, since the completion path will
-	 * have done that for us.
-	 */
-	if (must_wait || work == &work_stack) {
-		bdi_wait_on_work_clear(work);
-		if (work != &work_stack)
-			call_rcu(&work->rcu_head, bdi_work_free);
-	}
+	bdi_queue_work(wbc->bdi, &work);
+	bdi_wait_on_work_clear(&work);
 }
 
 /*