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

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Date:   Mon, 15 Apr 2019 14:55:03 +0000
From:   Kirill Smelkov <kirr@...edi.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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)

Hi Sebastian,

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:
> > Hello everyone,
> Hi,
> 
> > On Sat, Apr 13, 2019 at 06:50:57PM +0200, Julia Lawall wrote:
> > > Hello,
> > > 
> > > Kirill will explain about this issue.
> > 
> > pci/switchtec switching to stream_open is already queued to merge
> > window and it was acked by Logan Gunthorpe:
> > 
> >         https://lore.kernel.org/lkml/CAHk-=wgqgN5j1ZWnyVLqqoyU=CCWTYOko3MDyU8L_5e21KvHAg@mail.gmail.com/
> >         https://lab.nexedi.com/kirr/linux/commit/edaeb4101860
> > 
> > ( there are too many Cc's in that patch and email with it and reply-all to
> >   it did not get into mailing list probably due to being considered as spam )
> > 
> > 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

Kirill