Re: [PATCH] blk-mq: Put driver tag in blk_mq_dispatch_rq_list() when no budget

[Bart Van Assche]

On 4/20/20 1:22 AM, John Garry wrote:
> On 18/04/2020 03:43, Bart Van Assche wrote:
> > On 2020-04-16 04:18, John Garry wrote:
> > > If in blk_mq_dispatch_rq_list() we find no budget, then we break of the
> > > dispatch loop, but the request may keep the driver tag, evaulated
> > > in 'nxt' in the previous loop iteration.
> > >
> > > Fix by putting the driver tag for that request.
> > >
> > > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > >
> > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > index 8e56884fd2e9..a7785df2c944 100644
> > > --- a/block/blk-mq.c
> > > +++ b/block/blk-mq.c
> > > @@ -1222,8 +1222,10 @@ bool blk_mq_dispatch_rq_list(struct 
> > > request_queue *q, struct list_head *list,
> > > ÂÂÂÂÂÂÂÂÂ rq = list_first_entry(list, struct request, queuelist);
> > > ÂÂÂÂÂÂÂÂÂ hctx = rq->mq_hctx;
> > > -ÂÂÂÂÂÂÂ if (!got_budget && !blk_mq_get_dispatch_budget(hctx))
> > > +ÂÂÂÂÂÂÂ if (!got_budget && !blk_mq_get_dispatch_budget(hctx)) {
> > > +ÂÂÂÂÂÂÂÂÂÂÂ blk_mq_put_driver_tag(rq);
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
> > > +ÂÂÂÂÂÂÂ }
> > > ÂÂÂÂÂÂÂÂÂ if (!blk_mq_get_driver_tag(rq)) {
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ /*
> >
> > Is this something that can only happen if q->mq_ops->queue_rq(hctx, &bd)
> > returns another value than BLK_STS_OK, BLK_STS_RESOURCE and
> > BLK_STS_DEV_RESOURCE? 
>
> Right, as that case is handled in blk_mq_handle_dev_resource()
>
> If so, please add a comment in the source code
> > that explains this.
>
> So important that we should now do this in an extra patch?

Probably not, unless someone else feels strongly about this.

Thanks,

Bart.