linux-kernel - Re: [PATCH 2/6] g_NCR5380: Test the IRQ before accepting it

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Date:   Thu, 3 Nov 2016 13:17:04 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Ondrej Zary <linux@...nbow-software.org>
cc:     Christoph Hellwig <hch@...radead.org>, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] g_NCR5380: Test the IRQ before accepting it

On Wed, 2 Nov 2016, Ondrej Zary wrote:

> On Wednesday 02 November 2016 08:45:26 Finn Thain wrote:
> > On Mon, 31 Oct 2016, Ondrej Zary wrote:
> > > Trigger an IRQ first with a test IRQ handler to find out if it 
> > > really works. Disable the IRQ if not.
> > >
> > > This prevents hang when incorrect IRQ was specified by user.
> >
> > Once again, how does it cause a hang?
> 
> Kernel scans the bus, finds a HDD, then attempts to read MBR. modprobe 
> process is stuck but the system is still running. Then the transfer 
> probably times out and everything locks up hard, even fbcon cursor stops 
> blinking. I guess that kernel is trying to abort or reset.

I don't think this issue relates to the patch, because the chip irq is not 
needed for exception handling.

A backtrace from the soft lockup detector should help explain this.

> BTW. rescan-scsi-bus also causes hang, anytime, even without IRQ.

I would try "scsi_logging_level -s -a 7" to find out what is going on 
during the bus scan (for modprobe or rescan-scsi-bus).

The polling loops in generic_NCR5380_pread/pwrite can cause a lockup 
because they lack timeouts. Better to call NCR5380_poll_politely, as in 
macscsi_pread/pwrite.

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