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

Daniel Vetter
2011-11-21T16:02:44Z

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@ffwll.ch
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