RE: [PATCH V2] blk-mq: Set request mapping to NULL in blk_mq_put_driver_tag

From: Kashyap Desai <kashyap.desai@broadcom.com>
To: Jens Axboe <axboe@kernel.dk>,
	Bart Van Assche <bvanassche@acm.org>,
	linux-block <linux-block@vger.kernel.org>,
	linux-scsi <linux-scsi@vger.kernel.org>
Cc: Ming Lei <ming.lei@redhat.com>,
	Suganath Prabu Subramani  <suganath-prabu.subramani@broadcom.com>,
	Sreekanth Reddy <sreekanth.reddy@broadcom.com>,
	Sathya Prakash Veerichetty <sathya.prakash@broadcom.com>
Subject: RE: [PATCH V2] blk-mq: Set request mapping to NULL in blk_mq_put_driver_tag
Date: Tue, 18 Dec 2018 22:38:01 +0530	[thread overview]
Message-ID: <41bb993fdc71d02a884cc2b793403ca7@mail.gmail.com> (raw)
In-Reply-To: <ae741e1b-ec2d-075e-82b0-a1e09d0553fd@kernel.dk>

> >
> > Other block drivers (e.g. ib_srp, skd) do not need this to work
> > reliably.
> > It has been explained to you that the bug that you reported can be
> > fixed by modifying the mpt3sas driver. So why to fix this by modifying
> > the block layer? Additionally, what prevents that a race condition
> > occurs between the block layer clearing hctx->tags->rqs[rq->tag] and
> > scsi_host_find_tag() reading that same array element? I'm afraid that
> > this is an attempt to paper over a real problem instead of fixing the
> > root
> cause.
>
> I have to agree with Bart here, I just don't see how the mpt3sas use case
> is
> special. The change will paper over the issue in any case.

Hi Jens, Bart

One of the key requirement for iterating whole tagset  using
scsi_host_find_tag is to block scsi host. Once we are done that, we should
be good. No race condition is possible if that part is taken care.
Without this patch, if driver still receive scsi command from the
hctx->tags->rqs which is really not outstanding.  I am finding this is
common issue for many scsi low level drivers.

Just for example <fnic> - fnic_is_abts_pending() function has below code -

        for (tag = 0; tag < fnic->fnic_max_tag_id; tag++) {
                sc = scsi_host_find_tag(fnic->lport->host, tag);
                /*
                 * ignore this lun reset cmd or cmds that do not belong to
                 * this lun
                 */
                if (!sc || (lr_sc && (sc->device != lun_dev || sc ==
lr_sc)))
                        continue;

Above code also has similar exposure of kernel panic like <mpt3sas> driver
while accessing sc->device.

Panic is more obvious if we have add/removal of scsi device before looping
through scsi_host_find_tag().

Avoiding block layer changes is also attempted in <mpt3sas> but our problem
is to convert that code common for non-mq and mq.
Temporary to unblock this issue, We have fixed <mpt3sas> using driver
internals scsiio_tracker() instead of piggy back in scsi_command.

Kashyap

>
> --
> Jens Axboe