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

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Date:   Thu, 11 Oct 2018 09:55:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:     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>,
        Mike Christie <mchristi@...hat.com>,
        Hannes Reinecke <hare@...e.com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        "Bryant G. Ly" <bryantly@...ux.vnet.ibm.com>,
        Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH 2/2] target: Fix target_wait_for_sess_cmds breakage with
 active signals

On Wed, Oct 10, 2018 at 10:40:12PM -0700, Nicholas A. Bellinger wrote:
> Hey Peter & Co,
> 
> On Wed, 2018-10-10 at 10:43 +0200, Peter Zijlstra wrote:
> > On Wed, Oct 10, 2018 at 03:23:10AM +0000, 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.
> > > 
> > > For example, iscsi-target and iser-target issue SIGINT to all kthreads as
> > > part of session shutdown.  This has been the behaviour since day one.
> > 
> > Not knowing much context here; but does it make sense for those
> > kthreads to handle signals, ever? Most kthreads should be fine with
> > ignore_signals().
> > 
> 
> iscsi-target + ib-isert uses SIGINT amongst dedicated rx/tx connection
> kthreads to signal connection shutdown, requiring in-flight se_cmd I/O
> descriptors to be quiesced before making forward progress to release
> se_session.
> 
> By the point wait_event_lock_irq_timeout() is called in the example
> here, one of the two rx/tx connection kthreads has been stopped, and the
> other kthread is still processing shutdown.  So while historically the
> pending SIGINTs where not cleared (or ignored) during shutdown at this
> point, there is no reason why they could not be ignored for iscsi-target
> + ib-isert.
> 
> That said, pre commit 00d909a107 code always used wait_for_completion()
> and ignored pending signals.  As-is target_wait_for_sess_cmds() is
> called directly from fabric driver code and in one case also from
> user-space via configfs_write_file(), so AFAICT it does need
> TASK_UNINTERRUPTIBLE.
> 

Fair enough, thanks for the background. I'm always a bit wary when
kthreads need to deal with signals, but this seems OK.