linux-kernel - Re: [PATCH 2/2] target: Fix target_wait_for_sess

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Message-ID: <1539237365.6150.32.camel@haakon3.daterainc.com>
Date:   Wed, 10 Oct 2018 22:56:05 -0700
From:   "Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:     Mike Christie <mchristi@...hat.com>
Cc:     "Ly, Bryant" <bly@...alogicsoftware.com>,
        target-devel <target-devel@...r.kernel.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Hannes Reinecke <hare@...e.com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Bart Van Assche <bvanassche@....org>,
        bly@...alogicsoftware.com
Subject: Re: [PATCH 2/2] target: Fix target_wait_for_sess_cmds breakage with
 active signals

Hello MNC & Co,

On Wed, 2018-10-10 at 11:58 -0500, Mike Christie wrote:
> On 10/09/2018 10:23 PM, Nicholas A. Bellinger wrote:
> > From: Nicholas Bellinger <nab@...ux-iscsi.org>
> > 
> > With the addition of commit 00d909a107 in v4.19-rc, it incorrectly assumes no
> > signals will be pending for task_struct executing the normal session shutdown
> > and I/O quiesce code-path.
> > 

<SNIP>

> > diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
> > index 86c0156..fc3093d2 100644
> > --- a/drivers/target/target_core_transport.c
> > +++ b/drivers/target/target_core_transport.c
> > @@ -2754,7 +2754,7 @@ static void target_release_cmd_kref(struct kref *kref)
> >  	if (se_sess) {
> >  		spin_lock_irqsave(&se_sess->sess_cmd_lock, flags);
> >  		list_del_init(&se_cmd->se_cmd_list);
> > -		if (list_empty(&se_sess->sess_cmd_list))
> > +		if (se_sess->sess_tearing_down && list_empty(&se_sess->sess_cmd_list))
> 
> I think there is another issue with 00d909a107 and ibmvscsi_tgt.
> 
> The problem is that ibmvscsi_tgt never called
> target_sess_cmd_list_set_waiting. It only called
> target_wait_for_sess_cmds. So before 00d909a107 there was a bug in that
> driver and target_wait_for_sess_cmds never did what was intended because
> sess_wait_list would always be empty.
> 
> With 00d909a107, we no longer need to call
> target_sess_cmd_list_set_waiting to wait for outstanding commands, so
> for ibmvscsi_tgt will now wait for commands like we wanted. However, the
> commit added a WARN_ON that is hit if target_sess_cmd_list_set_waiting
> is not called, so we could hit that.
> 
> So I think we need to add a target_sess_cmd_list_set_waiting call in
> ibmvscsi_tgt to go along with your patch chunk above and make sure we do
> not trigger the WARN_ON.
> 

Nice catch.  :)

With target_wait_for_sess_cmd() usage pre 00d909a107 doing a list-splice
in target_sess_cmd_list_set_waiting(), this particular usage in
ibmvscsi_tgt has always been list_empty(&sess->sess_wait_list) = true
(eg: no se_cmd I/O is quiesced, because no se_cmd in sess_wait_list)
since commit 712db3eb in 4.9.y code.

That said, ibmvscsi_tgt usage is very similar to vhost/scsi in the
respect individual /sys/kernel/config/target/$FABRIC/$WWN/$TPGT/
endpoints used by VMs do not remove their I_T nexus while the VM is
active.

So AFAICT, ibmvscsi_tgt doesn't strictly need target_sess_wait_for_cmd()
at all if this is true.