linux-kernel - Re: [PATCH] pci/switchtec: fix stream

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Date:   Mon, 15 Apr 2019 17:20:22 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Kirill Smelkov <kirr@...edi.com>
Cc:     Julia Lawall <julia.lawall@...6.fr>, kbuild-all@...org,
        Kurt Schwemmer <kurt.schwemmer@...rosemi.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pci/switchtec: fix stream_open.cocci warnings (fwd)

On 2019-04-15 14:55:02 [+0000], Kirill Smelkov wrote:
> Hi Sebastian,
Hi Kirill,

> On Mon, Apr 15, 2019 at 04:38:57PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2019-04-13 17:00:59 [+0000], Kirill Smelkov wrote:
> > > stream_open.cocci was issuing only warning for pci/switchtec, but after
> > > 8a29a3bae2a2 ("pci/switchtec: Don't use completion's wait queue") they
> > > started to use wait_even_* inside read method and, since
> > > stream_open.cocci considers wait_event_* as blocking the warning became
> > > error. Previously it was completions there, but I added support for wait
> > > events only for simplicity.
> > 
> > why is wait_event_interruptible() treated differently compared to
> > wait_for_completion_interruptible()?
> 
> No particular reason. I just taught stream_open.cocci to consider
> only "wait_event_*" as blocking:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/coccinelle/api/stream_open.cocci?h=v5.1-rc5#n35
> 
> based on original /proc/xen/xenbus deadlock:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/xen/xenbus/xenbus_dev_frontend.c?h=v5.1-rc5#n135
> https://git.kernel.org/linus/581d21a2d02a
> 
> We can extend "a function that blocks" rule to cover other kernel
> primitives.
> 
> For the reference: the deadlock scenario is described in
> 
> https://git.kernel.org/linus/10dce8af3422

As far I understand the problem is when the ->read() callback waits for
the ->write() callback. The locking isn't changed by patch you
mentioned.
So extended might make sense. But then wait_event_* by itself in
->read() isn't a problem as long as its counter part isn't in ->write().
But yes, nice finding.

> Kirill

Sebastian