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

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

On 07/11/2014 03:02 PM, Daniel Vetter wrote:
> On Fri, Jul 11, 2014 at 12:06:02PM +0100, Chris Wilson wrote:
>> On Fri, Jul 11, 2014 at 12:00:26PM +0100, Tvrtko Ursulin wrote:
>>>
>>> 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?
>>
>> A threaded client. One thread using userptr, the other doing munmap or
>> free. Given enough embarrassment, it will happen every time.
>
> Can I have an igt please to confirm this and ensure it stays fixed?

Sure, give me a day or two.

Tvrtko