Re: [PATCH v2] drm/i915/gtt: Serialise both updates to PDE and our shadow

[Matthew Auld <matthew.auld@intel.com>]

On 17/06/2019 12:20, Chris Wilson wrote:
> Currently, we perform a locked update of the shadow entry when
> allocating a page directory entry such that if two clients are
> concurrently allocating neighbouring ranges we only insert one new entry
> for the pair of them. However, we also need to serialise both clients
> wrt to the actual entry in the HW table, or else we may allow one client
> or even a third client to proceed ahead of the HW write. My handwave
> before was that under the _pathological_ condition we would see the
> scratch entry instead of the expected entry, causing a temporary
> glitch. That starvation condition will eventually show up in practice, so
> fix it.
> 
> The reason for the previous cheat was to avoid having to free the extra
> allocation while under the spinlock. Now, we keep the extra entry
> allocated until the end instead.
> 
> v2: Fix error paths for gen6
> 
> Fixes: 1d1b5490b91c ("drm/i915/gtt: Replace struct_mutex serialisation for allocation")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
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