linux-kernel - Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and queue is idle

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Date:   Thu, 7 Dec 2017 21:06:58 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "ming.lei@...hat.com" <ming.lei@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "axboe@...com" <axboe@...com>, "hare@...e.com" <hare@...e.com>,
        "holger@...lied-asynchrony.com" <holger@...lied-asynchrony.com>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>
Subject: Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and
 queue is idle

On Wed, 2017-12-06 at 00:28 +0800, Ming Lei wrote:
> On Tue, Dec 05, 2017 at 04:08:20PM +0000, Bart Van Assche wrote:
> > On Tue, 2017-12-05 at 15:52 +0800, Ming Lei wrote:
> > > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > > index db9556662e27..1816dd8259b3 100644
> > > --- a/drivers/scsi/scsi_lib.c
> > > +++ b/drivers/scsi/scsi_lib.c
> > > @@ -1967,6 +1967,8 @@ static bool scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx)
> > >  out_put_device:
> > >  	put_device(&sdev->sdev_gendev);
> > >  out:
> > > +	if (atomic_read(&sdev->device_busy) == 0 && !scsi_device_blocked(sdev))
> > > +		blk_mq_delay_run_hw_queue(hctx, SCSI_QUEUE_DELAY);
> > >  	return false;
> > >  }
> > 
> > This cannot work since multiple threads can call scsi_mq_get_budget()
> 
> That is exactly the way we are handling these cases before 0df21c86bdbf(scsi:
> implement .get_budget and .put_budget for blk-mq), so if it can't work,
> that is not fault of commit 0df21c86bdbf.
> 
> > concurrently and hence it can happen that none of them sees
> > atomic_read(&sdev->device_busy) == 0. BTW, the above patch is something I
> 
> If there is concurrent .get_budget(), one of them will see the counter
> becoming zero finally because each sdev->device_busy is inc/dec
> atomically. Or scsi_dev_queue_ready() return true.
> 
> Anyway, we need this patch to avoid possible regression. If you think
> there are bugs in blk-mq RESTART, just root cause and and fix it.

Hello Ming,

When I looked at the patch at the start of this thread for the first time I
got frustrated because I didn't see how this patch could fix the queue stall
I ran into myself. Today I started realizing that what Holger reported is
probably another issue than what I ran into myself. Since this patch by
itself looks now useful to me:

Reviewed-by: Bart Van Assche <bart.vanassche@....com>

BTW, do you think the patch at the start of this thread also fixes the issue
that resulted in commit 826a70a08b12 ("SCSI: don't get target/host busy_count
in scsi_mq_get_budget()")? Do you think we still need that patch?

Bart.