linux-kernel - Re: Signal handling in a page fault handler

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Message-ID: <20180403151220.GW3881@phenom.ffwll.local>
Date:   Tue, 3 Apr 2018 17:12:20 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Thomas Hellstrom <thomas@...pmail.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        Souptick Joarder <jrdr.linux@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: Signal handling in a page fault handler

On Tue, Apr 03, 2018 at 07:48:29AM -0700, Matthew Wilcox wrote:
> On Tue, Apr 03, 2018 at 03:12:35PM +0200, Thomas Hellstrom wrote:
> > I think the TTM page fault handler originally set the standard for this.
> > First, IMO any critical section that waits for the GPU (like typically the
> > page fault handler does), should be locked at least killable. The need for
> > interruptible locks came from the X server's silken mouse relying on signals
> > for smooth mouse operations: You didn't want the X server to be stuck in the
> > kernel waiting for GPU completion when it should handle the cursor move
> > request.. Now that doesn't seem to be the case anymore but to reiterate
> > Chris' question, why would the signal persist once returned to user-space?
> 
> Yeah, you graphics people have had to deal with much more recalcitrant
> hardware than most of the rest of us ... and less reasonable user
> expectations ("My graphics card was doing something and I expected
> everything else to keep going" vs "My hard drive died and my kernel
> paniced, oh well.")
> 
> I don't know exactly how the signal code works at the delivery end;
> I'm not sure when TIF_SIGPENDING gets cleared.  I just get concerned
> when I see one bit of kernel code doing things in a very complicated
> and careful manner and another bit of kernel code blithely assuming
> that everything's going to be OK.

I think you last line pretty much sums up the proper attitude when writing
gpu drivers:

https://i.imgflip.com/27nm7w.jpg

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch