[RFC][PATCH 05/16] writeback: fix possible race when creating bdi threads

[Artem Bityutskiy <dedekind1@gmail.com>]

From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

This patch fixes a very unlikely race condition on the error path.
When bdi thread creation fails, 'bdi->wb.task' may temporary, for
very short time, contain an error code. If at the same time someone
submits a task to this bdi, the following code will oops
in 'bdi_queue_work()'

  if (wb->task)
	  wake_up_process(wb->task);

Fix this by introducing a temporary variable 'task' and storing
the possible error code there, so that 'wb->task' will never
take bogus values.

Note, this race is very unlikely and I never hit it, so it is
theoretical, but nevertheless worth fixing.

This patch also merges 2 comments which were previously separate.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 mm/backing-dev.c |   28 +++++++++++-----------------
 1 files changed, 11 insertions(+), 17 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index a445ff0..8be2e13 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -332,7 +332,7 @@ static int bdi_forker_thread(void *ptr)
 
 	for (;;) {
 		struct backing_dev_info *bdi, *tmp;
-		struct bdi_writeback *wb;
+		struct task_struct *task;
 
 		/*
 		 * Temporary measure, we want to make sure we don't see
@@ -383,29 +383,23 @@ static int bdi_forker_thread(void *ptr)
 		list_del_init(&bdi->bdi_list);
 		spin_unlock_bh(&bdi_lock);
 
-		wb = &bdi->wb;
-		wb->task = kthread_run(bdi_writeback_thread, wb, "flush-%s",
-					dev_name(bdi->dev));
-		/*
-		 * If thread creation fails, then readd the bdi to
-		 * the pending list and force writeout of the bdi
-		 * from this forker thread. That will free some memory
-		 * and we can try again.
-		 */
-		if (IS_ERR(wb->task)) {
-			wb->task = NULL;
-
+		task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s",
+ 					dev_name(bdi->dev));
+		if (IS_ERR(task)) {
 			/*
-			 * Add this 'bdi' to the back, so we get
-			 * a chance to flush other bdi's to free
-			 * memory.
+			 * If thread creation fails, then readd the bdi back to
+			 * the list and force writeout of the bdi from this
+			 * forker thread. That will free some memory and we can
+			 * try again. Add it to the tail so we get a chance to
+			 * flush other bdi's to free memory.
 			 */
 			spin_lock_bh(&bdi_lock);
 			list_add_tail(&bdi->bdi_list, &bdi_pending_list);
 			spin_unlock_bh(&bdi_lock);
 
 			bdi_flush_io(bdi);
-		}
+		} else
+			bdi->wb.task = task;
 	}
 
 	return 0;
-- 
1.7.1.1