[PATCHv5 02/15] writeback: fix possible race when creating bdi threads

Artem Bityutskiy
2010-07-25T08:30:46Z

This patch fixes a very unlikely race condition on the bdi forker thread error
path: when bdi thread creation fails, 'bdi->wb.task' may contain the error code
for a short period of time. If at the same time someone submits a work to this
bdi, we can end up with an oops 'bdi_queue_work()' while executing
'wake_up_process(wb->task)'.

This patch fixes the issue by introducing a temporary variable 'task' and
storing the possible error code there, so that 'wb->task' would never take
erroneous 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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
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 4e9ed2a..327e36d 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -331,8 +331,8 @@ static int bdi_forker_thread(void *ptr)
set_user_nice(current, 0);

for (;;) {
+ struct task_struct *task;
struct backing_dev_info *bdi, *tmp;
- struct bdi_writeback *wb;

/*
* 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