On Thu, 17 Sep 2015 08:39:44 -0400
Jeff Layton <jlayton@xxxxxxxxxxxxxxx> wrote:
> 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 immediately schedule the work to run if it's not already,
> and block until the workqueue job completes.
>
> Signed-off-by: Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
> ---
> fs/file_table.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
It should be noted that the only current user of flush_delayed_fput()
can call it before the workqueue threads are ever started.
Looking at the code, I *think* this will still do the right thing --
block until those threads are started and then flush the work as usual,
but do let me know if I've misread it.
> diff --git a/fs/file_table.c b/fs/file_table.c
> index ad17e05ebf95..52cc6803c07a 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -244,6 +244,8 @@ 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
> @@ -256,11 +258,9 @@ 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)) {
--
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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