On Wed, 31 Aug 2011 10:41:49 +0530
Rajan Aggarwal <rajan.aggarwal85@xxxxxxxxx> wrote:
> The bdi_writeback_thread function does not use spin_lock to
> see if the work_list is empty.
>
> If the list is not empty, and if an interrupt happens before we
> set the current->state to TASK_RUNNING then we could be stuck in
> a schedule() due to kernel preemption.
>
> This patch acquires and releases the wb_lock to avoid this scenario.
>
> Signed-off-by: Rajan Aggarwal <rajan.aggarwal85@xxxxxxxxx>
> ---
> fs/fs-writeback.c | 3 +++
> 1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 04cf3b9..e333898 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -936,11 +936,14 @@ int bdi_writeback_thread(void *data)
> if (pages_written)
> wb->last_active = jiffies;
>
> + spin_lock_bh(&bdi->wb_lock);
> set_current_state(TASK_INTERRUPTIBLE);
> if (!list_empty(&bdi->work_list) || kthread_should_stop()) {
> __set_current_state(TASK_RUNNING);
> + spin_unlock_bh(&bdi->wb_lock);
> continue;
> }
> + spin_unlock_bh(&bdi->wb_lock);
>
> if (wb_has_dirty_io(wb) && dirty_writeback_interval)
> schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
I don't see anything particularly wrong with the current code. If a
task gets preempted while in state TASK_INTERRUPTIBLE then it will
still be in that state when that task resumes running.
There might be some cross-CPU memory ordering issues in that code. If
so, the effects would be:
a) list_empty() falsely thought to return "false": the thread will
do one additional pointless loop and will then sleep.
b) list_empty() falsely thought to return "true": the thread will
prematurely attempt to go to sleep, introducing a teent bit of
additional latency in rare cases. But I think this is a "can't
happen" because of the memory barrier in
set_current_state(TASK_INTERRUPTIBLE): if the task made this mistake
running list_empty() then it will now be in state TASK_RUNNING and
the schedule() calls will fall straight through. I think.
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