Re: [Intel-gfx] [PATCH v2] drm/i915: Force wmb() on using GTT io_mapping_map_wc

[Chris Wilson <chris@chris-wilson.co.uk>]

On Tue, May 26, 2015 at 10:01:24AM +0200, Daniel Vetter wrote:
> On Mon, May 25, 2015 at 06:48:44PM +0100, Chris Wilson wrote:
> > Since the advent of mmap(wc), where we reused the same cache domain for
> > WC and GTT paths (oh, how I regret that double-edged advice), we need to
> > be extra cautious when using GTT iomap_wc internally. Since userspace maybe
> > modifying through the mmap(wc) and we then modify modifying through an
> > aliased WC path through the GTT, those writes may overlap and not be
> > visible to the other path. Easiest to trigger appears to be write the
> > batch through mmap(wc) and then attempt to perform reloc the GTT,
> > corruption quickly ensues.
> > 
> > v2: Be stricter and do a full mb() in case we are reading through one
> > path and about to write through the second path. Also, apply the
> > barriers on transitioning into the GTT domain as well to cover the white
> > lie asychronous cases.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Akash Goel <akash.goel@intel.com>
> > Cc: stable@vger.kernel.org
> 
> There are a lot more mb() and wmb() in the gem code. I count execlist,
> legacy rings, pwrite, set_domain and finish_gtt.

You have more than I have...
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre