[PATCH] Ensure blitter quiescience before reading pixels from the framebuffer

[Bernardo Innocenti]

Alex Deucher wrote:

> FWIW, I've seen similar issues with EXA on savage.

The codepath I'm fixing here cannot be responsible for
that, but it seems that X is missing protection against
such races in several places.

In the AMD driver, the problem was in the Composite hook.
But I see the Savage driver does not even implement it.

There's a readfb() function that lacks protection, but
apparently it's dead code.

-- 
   // Bernardo Innocenti
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