linux-kernel - Re: [Intel-gfx] [PATCH 02/13] drm/i915: rewrite shmem_pwrite_slow to use copy_from

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Message-ID: <20111121160244.GA3807@phenom.ffwll.local>
Date:	Mon, 21 Nov 2011 17:02:44 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Ben Widawsky <ben@...dawsk.net>
Cc:	Daniel Vetter <daniel.vetter@...ll.ch>,
	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH 02/13] drm/i915: rewrite shmem_pwrite_slow to
 use copy_from_user

On Sun, Nov 20, 2011 at 09:56:32PM -0800, Ben Widawsky wrote:
[snip the patch]
> Bikeshed, but I would much prefer a #define for the swizzle
> bit/cacheline size.

I've looked at this stuff way too long, so I'm biased, but 64 = cacheline
= dram fetch size = 1 << 64 feels about as natural for me as 4096 =
PAGE_SIZE ...

[snip the patch]

> I must be missing something obvious here...
> Can you explain how this can possibly be considered safe without holding
> struct_mutex?

That's the reason why the commit msg goes through every case and explains
why I think it's safe. The large thing here is that we need to drop the
mutex when calling copy_*_user (at least in the non-atomic slow-paths)
because otherwise we might deadlock with our own pagefault handler.
-Daniel
-- 
Daniel Vetter
Mail: daniel@...ll.ch
Mobile: +41 (0)79 365 57 48
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