From: Bart Van Assche <Bart.VanAssche@wdc.com>
To: "ming.lei@redhat.com" <ming.lei@redhat.com>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "hch@infradead.org" <hch@infradead.org>,
        "martin.petersen@oracle.com" <martin.petersen@oracle.com>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "axboe@fb.com" <axboe@fb.com>, "hare@suse.com" <hare@suse.com>,
        "holger@applied-asynchrony.com" <holger@applied-asynchrony.com>,
        "jejb@linux.vnet.ibm.com" <jejb@linux.vnet.ibm.com>
Subject: Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and
 queue is idle
Thread-Topic: [PATCH] SCSI: run queue if SCSI device queue isn't ready and
 queue is idle
Thread-Index: AQHTbZ4g7T3yQq9yCkKeO51Qk7DLeaM060uAgAAFpYCAA3J1gA==
Date: Thu, 7 Dec 2017 21:06:58 +0000
Message-ID: <1512680817.2624.29.camel@wdc.com>
References: <20171205075256.10319-1-ming.lei@redhat.com>
         <1512490099.2660.6.camel@sandisk.com> <20171205162825.GA23788@ming.t460p>
In-Reply-To: <20171205162825.GA23788@ming.t460p>
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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@wdc.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.