[PATCH] fs: have flush_delayed_fput flush the workqueue job

[Jeff Layton <jlayton@poochiereds.net>]

I think there's a potential race in flush_delayed_fput. A kthread does
an fput() and that file gets added to the list and the delayed work is
scheduled. More than 1 jiffy passes, and the workqueue thread picks up
the work and starts running it. Then the kthread calls
flush_delayed_work.  It sees that the list is empty and returns
immediately, even though the __fput for its file may not have run yet.

Close this by making flush_delayed_fput use flush_delayed_work instead,
which should just block until the workqueue job completes if it's
already running.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
---
 fs/file_table.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/file_table.c b/fs/file_table.c
index 7f9d407c7595..f4833af62eae 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -243,6 +243,8 @@ static void ____fput(struct callback_head *work)
 	__fput(container_of(work, struct file, f_u.fu_rcuhead));
 }
 
+static DECLARE_DELAYED_WORK(delayed_fput_work, delayed_fput);
+
 /*
  * If kernel thread really needs to have the final fput() it has done
  * to complete, call this.  The only user right now is the boot - we
@@ -255,11 +257,9 @@ static void ____fput(struct callback_head *work)
  */
 void flush_delayed_fput(void)
 {
-	delayed_fput(NULL);
+	flush_delayed_work(&delayed_fput_work);
 }
 
-static DECLARE_DELAYED_WORK(delayed_fput_work, delayed_fput);
-
 void fput(struct file *file)
 {
 	if (atomic_long_dec_and_test(&file->f_count)) {
-- 
2.4.3