[Bug 91611] [BDW, BSW] arb_shader_image_load_store.execution.basic-imagestore-from-uniform intermittent

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91611

--- Comment #1 from Francisco Jerez <currojerez at riseup.net> ---
I've been looking into this today and I don't think that this test could
actually be causing the failure you've seen, it's pretty much trivial, most
likely it was run concurrently with some other test that hung the GPU causing
it to misrender (can you confirm if there was a GPU hang message in your kernel
logs when the failure occurred?).

I've managed to reproduce an intermittent hang on BDW by running piglit in a
loop, but it comes from the spec at arb_shader_image_load_store@host-mem-barrier
Indirect/RaW subtest.  It seems rather difficult to reproduce, it often takes
dozens of runs until the first failure occurs, which is why it didn't catch my
eye until now.

It looks like the hang is caused by some interaction between context switches
with the hardware UAV coherency function on the VS stage, which leads the VS to
stall indefinitely for some reason I don't fully understand yet.  The attached
hack should sidestep the issue.

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