linux-kernel - Re: [PATCH 2/2] cciss: Fix SCSI device reset handler

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Date:	Tue, 2 Jun 2009 13:40:25 -0500
From:	scameron@...rdog.cca.cpqcorp.net
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	mikem@...rdog.cca.cpqcorp.net
Subject: Re: [PATCH 2/2] cciss: Fix SCSI device reset handler

On Tue, Jun 02, 2009 at 08:28:31PM +0200, Jens Axboe wrote:
> On Tue, Jun 02 2009, scameron@...rdog.cca.cpqcorp.net wrote:
> > 
> > On Tue, Jun 02, 2009 at 07:58:14PM +0200, Jens Axboe wrote:
> > > On Tue, Jun 02 2009, Andrew Morton wrote:
> > > > On Tue, 2 Jun 2009 14:50:11 +0200
> > > > Jens Axboe <jens.axboe@...cle.com> wrote:
> > > > 
> > > > > On Fri, May 29 2009, Andrew Morton wrote:
> > > > > > On Wed, 27 May 2009 15:30:07 -0500
> > > > > > scameron@...rdog.cca.cpqcorp.net wrote:
> > > > > > 
> > > > > > > +static int wait_for_device_to_become_ready(ctlr_info_t *h,
> > > > > > > +	unsigned char lunaddr[])
> > > > > > > +{
> > > > > > > +	int rc;
> > > > > > > +	int count = 0;
> > > > > > > +	int waittime = HZ;
> > > > > > > +	CommandList_struct *c;
> > > > > > > +
> > > > > > > +	c = cmd_alloc(h, 1);
> > > > > > > +	if (!c) {
> > > > > > > +		printk(KERN_WARNING "cciss%d: out of memory in "
> > > > > > > +			"wait_for_device_to_become_ready.\n", h->ctlr);
> > > > > > > +		return IO_ERROR;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	/* Send test unit ready until device ready, or give up. */
> > > > > > > +	while (count < 20) {
> > > > > > > +
> > > > > > > +		/* Wait for a bit.  do this first, because if we send
> > > > > > > +		 * the TUR right away, the reset will just abort it.
> > > > > > > +		 */
> > > > > > > +		set_current_state(TASK_INTERRUPTIBLE);
> > > > > > > +		schedule_timeout(waittime);
> > > > > > 
> > > > > > That's schedule_timeout_interruptible().
> > > > > > 
> > > > > > The problem with interruptible sleeps of this nature is that they are
> > > > > > no-ops if the calling process happens to have signal_pending().  I
> > > > > > suspect that this condition will break your driver.
> > > > > > 
> > > > > > If so, switching to schedule_timeout_uninterruptible() will unbreak it.
> > > > > 
> > > > > I added Stephens patch and your fixup.
> > > > 
> > > > My cciss-fix-scsi-device-reset-handler-fix.patch was a simple cleanup -
> > > > it uses schedule_timeout_interruptible().
> > > > 
> > > > I believe that this should be changed to
> > > > schedule_timeout_uninterruptible() for the above reasons, but the cciss
> > > > guys fell asleep on me.
> > > 
> > > It's an improvement, none the less. And I bet it should just be
> > > uninterruptible sleep, unless it has a good reason to accept signals.
> > > Mike? Stephen?
> > > 
> > > -- 
> > > Jens Axboe
> > > 
> > 
> > Sorry for the slow reply.
> > 
> > No good reason that I know to accept signals, I'll defer to your
> > judgement on that.  When I wrote that schedule_timeout_... line,
> > I was vaguely wondering if it was quite right.
> > 
> > That being said, I'm working on a set of patches to make the cciss
> > SCSI error handling stuff work with interrupts enabled, which
> > means making similar changes to sendcmd_withirq() as I already
> > did to sendcmd() among some other stuff.   I didn't notice until
> > just a few days ago that since sometime in 2.4 kernels you no longer 
> > need to do the SCSI error handling with interrupts disabled as
> > in 2.2 kernels.  Mike asked me why we did it with interrupts
> > disabled, and I went looking in the docs to try to find where
> > it said we needed to do that, and... oh, that requirement is
> > gone.
> > 
> > So, if I rewrite the stuff to work with interrupts enabled, 
> > would that change which kind of schedule_timeout() should be used?
> > Or is that unrelated, and it depends whether you plan to do
> > something with signals?  I'm not all that clear when to use
> > one vs. tha other.
> 
> For short sleeps and sleeps that are outside of direct process context,
> an uninterruptible sleep is typically the right thing to do. Since this
> function is invoked from the scsi eh, doing signal enabled sleeps are
> pointless.

Ok, thanks.

I think in a few days I should have five or six more patches to clean up
some of the sendcmd_() junk a little bit, and get the SCSI error handling
stuff working with interrupts enabled, and I think that may allow the weird thing
where we stow away completed commands that sendcmd() scoops up inadvertently
for later processing to be removed, which would be nice.

-- steve

> 
> -- 
> Jens Axboe
> 
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