Re: [PATCH 2/2] drm/i915: Initialise userptr mmu_notifier serial to 1

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 07/11/2014 11:28 AM, Chris Wilson wrote:
> During the range invalidate, we walk the list of buffers associated with
> the mmu_notifer and find the ones that overlap the range. An
> optimisation is made to speed up the iteration by assuming the previous
> iter is still valid whilst the tree is unmodified. This exposes a bug
> when a range invalidate is triggered after we have just created the
> mmu_notifier, but before attaching any buffers. In that case, we presume
> we have an unmodified list and start walking from the last iter which is
> NULL. Oops.
>
> The easiest fix is then to initialise the serial of the tree to 1.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_gem_userptr.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index 7c38f50014db..8e9e91029aed 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -197,7 +197,7 @@ i915_mmu_notifier_get(struct drm_device *dev, struct mm_struct *mm)
>   	mmu->mm = mm;
>   	mmu->objects = RB_ROOT;
>   	mmu->count = 0;
> -	mmu->serial = 0;
> +	mmu->serial = 1;
>   	INIT_LIST_HEAD(&mmu->linear);
>   	mmu->is_linear = false;
>

Looks good to me and I think safe to merge in any case, so:

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

But it will be interesting to know what code managed to trigger this 
race, because as we discussed on IRC it would indicate some pretty wild 
userspace behaviour. Or lack of imagination on our part?

Tvrtko