Re: [Intel-gfx] [PATCH 16/26] drm/i915: use new iterator in i915_gem_object_wait_reservation v2

From: "Christian König" <ckoenig.leichtzumerken@gmail.com>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
	linaro-mm-sig@lists.linaro.org, dri-devel@lists.freedesktop.org,
	linux-media@vger.kernel.org, intel-gfx@lists.freedesktop.org
Cc: daniel@ffwll.ch
Subject: Re: [Intel-gfx] [PATCH 16/26] drm/i915: use new iterator in i915_gem_object_wait_reservation v2
Date: Tue, 21 Sep 2021 19:35:53 +0200	[thread overview]
Message-ID: <e66dd0dc-02a6-7437-ab98-dff4e8ff8de3@gmail.com> (raw)
In-Reply-To: <e0954bdd-2183-f662-8192-c44f931c602b@linux.intel.com>

Am 20.09.21 um 12:00 schrieb Tvrtko Ursulin:
>
> On 17/09/2021 13:35, Christian König wrote:
>> Simplifying the code a bit.
>>
>> v2: add missing rcu read unlock.
>>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
>> ---
>>   drivers/gpu/drm/i915/gem/i915_gem_wait.c | 57 ++++++------------------
>>   1 file changed, 14 insertions(+), 43 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_wait.c 
>> b/drivers/gpu/drm/i915/gem/i915_gem_wait.c
>> index f909aaa09d9c..e416cf528635 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_wait.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_wait.c
>> @@ -37,55 +37,26 @@ i915_gem_object_wait_reservation(struct dma_resv 
>> *resv,
>>                    unsigned int flags,
>>                    long timeout)
>>   {
>> -    struct dma_fence *excl;
>> -    bool prune_fences = false;
>> -
>> -    if (flags & I915_WAIT_ALL) {
>> -        struct dma_fence **shared;
>> -        unsigned int count, i;
>> -        int ret;
>> -
>> -        ret = dma_resv_get_fences(resv, &excl, &count, &shared);
>> -        if (ret)
>> -            return ret;
>> -
>> -        for (i = 0; i < count; i++) {
>> -            timeout = i915_gem_object_wait_fence(shared[i],
>> -                                 flags, timeout);
>> -            if (timeout < 0)
>> -                break;
>> -
>> -            dma_fence_put(shared[i]);
>> -        }
>> -
>> -        for (; i < count; i++)
>> -            dma_fence_put(shared[i]);
>> -        kfree(shared);
>> -
>> -        /*
>> -         * If both shared fences and an exclusive fence exist,
>> -         * then by construction the shared fences must be later
>> -         * than the exclusive fence. If we successfully wait for
>> -         * all the shared fences, we know that the exclusive fence
>> -         * must all be signaled. If all the shared fences are
>> -         * signaled, we can prune the array and recover the
>> -         * floating references on the fences/requests.
>> -         */
>> -        prune_fences = count && timeout >= 0;
>> -    } else {
>> -        excl = dma_resv_get_excl_unlocked(resv);
>> +    struct dma_resv_iter cursor;
>> +    struct dma_fence *fence;
>> +
>> +    rcu_read_lock();
>> +    dma_resv_iter_begin(&cursor, resv, flags & I915_WAIT_ALL);
>> +    dma_resv_for_each_fence_unlocked(&cursor, fence) {
>> +        rcu_read_unlock();
>> +        timeout = i915_gem_object_wait_fence(fence, flags, timeout);
>
> Converting this one could be problematic. It's the wait ioctl which 
> used to grab an atomic snapshot and wait for that rendering to 
> complete. With this change I think it has the potential to run forever 
> keeps catching new activity against the same object.
>
> I am not sure whether or not the difference is relevant for how 
> userspace uses it but I think needs discussion.

It was years ago, but IIRC we had the same discussion for the 
dma_resv_wait_timeout() function and the result was that this is not a 
valid use case and waiting forever if you see new work over and over 
again is a valid result.

Let me double check the history of this code here as well.

> Hm actually there are internal callers as well, and at least some of 
> those have the object locked. Would a wider refactoring to separate 
> those into buckets (locked vs unlocked) make sense?

Yes definitely.

Regards,
Christian.

>
> Regards,
>
> Tvrtko
>
>
>> +        rcu_read_lock();
>> +        if (timeout < 0)
>> +            break;
>>       }
>> -
>> -    if (excl && timeout >= 0)
>> -        timeout = i915_gem_object_wait_fence(excl, flags, timeout);
>> -
>> -    dma_fence_put(excl);
>> +    dma_resv_iter_end(&cursor);
>> +    rcu_read_unlock();
>>         /*
>>        * Opportunistically prune the fences iff we know they have 
>> *all* been
>>        * signaled.
>>        */
>> -    if (prune_fences)
>> +    if (timeout > 0)
>>           dma_resv_prune(resv);
>>         return timeout;
>>