Re: [PATCH V2 1/5] block: introduce blk_quiesce_timeout() and blk_unquiesce_timeout()

Ming Lei
2018-05-02T03:33:43Z

On Wed, May 02, 2018 at 10:23:35AM +0800, jianchao.wang wrote:
> Hi ming
>
> On 04/29/2018 11:41 PM, Ming Lei wrote:
> >
> > +static void __blk_unquiesce_timeout(struct request_queue *q)
> > +{
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(q->queue_lock, flags);
> > + q->timeout_off = false;
> > + spin_unlock_irqrestore(q->queue_lock, flags);
> > +}
> > +
> > +void blk_unquiesce_timeout(struct request_queue *q)
> > +{
> > + __blk_unquiesce_timeout(q);
> > + mod_timer(&q->timeout, jiffies + q->rq_timeout);
> > +}
> > +EXPORT_SYMBOL(blk_unquiesce_timeout);
> > +
> > +void blk_quiesce_timeout(struct request_queue *q)
> > +{
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(q->queue_lock, flags);
> > + q->timeout_off = true;
> > + spin_unlock_irqrestore(q->queue_lock, flags);
> > +
> > + del_timer_sync(&q->timeout);
> > + cancel_work_sync(&q->timeout_work);
> > +}
> > +EXPORT_SYMBOL(blk_quiesce_timeout);
> > +
> > /**
> > * blk_sync_queue - cancel any pending callbacks on a queue
> > * @q: the queue
> > @@ -408,8 +437,7 @@ EXPORT_SYMBOL(blk_stop_queue);
> > */
> > void blk_sync_queue(struct request_queue *q)
> > {
> > - del_timer_sync(&q->timeout);
> > - cancel_work_sync(&q->timeout_work);
> > + blk_quiesce_timeout(q);
> >
> > if (q->mq_ops) {
> > struct blk_mq_hw_ctx *hctx;
> > @@ -421,6 +449,8 @@ void blk_sync_queue(struct request_queue *q)
> > } else {
> > cancel_delayed_work_sync(&q->delay_work);
> > }
> > +
> > + __blk_unquiesce_timeout(q);
> > }
> > EXPORT_SYMBOL(blk_sync_queue);
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 0dc9e341c2a7..890dd3b3e3e1 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -901,6 +901,15 @@ static void blk_mq_timeout_work(struct work_struct *work)
> > };
> > struct blk_mq_hw_ctx *hctx;
> > int i;
> > + bool timeout_off;
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(q->queue_lock, flags);
> > + timeout_off = q->timeout_off;
> > + spin_unlock_irqrestore(q->queue_lock, flags);
> > +
> > + if (timeout_off)
> > + return;
>
> Looks like there is still race as following:
>
> blk_quiesce_timeout blk_mq_timeout_work
> -> timeout_off = q->timeout_off; //still a false
> -> q->timeout_off = true;
> -> del_timer_sync(&q->timeout);
> -> mod_timer
> -> cancel_work_sync(&q->timeout_work);

No, there isn't such race, the 'mod_timer' doesn't make a difference
because 'q->timeout_off' will be visible in new work func after
cancel_work_sync() returns. So even the timer is expired, work func
still returns immediately.

That means del_timer_sync() isn't absolutely necessary too.

Thanks,
Ming