Re: [PATCH 8/8] dma-buf: nuke reservation_object seq number

From: "Koenig, Christian" <Christian.Koenig@amd.com>
To: "Chris Wilson" <chris@chris-wilson.co.uk>,
	"Christian König" <ckoenig.leichtzumerken@gmail.com>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	"linaro-mm-sig@lists.linaro.org" <linaro-mm-sig@lists.linaro.org>
Subject: Re: [PATCH 8/8] dma-buf: nuke reservation_object seq number
Date: Wed, 7 Aug 2019 13:05:08 +0000	[thread overview]
Message-ID: <be4c4454-4672-65d5-073b-b399aa43338c@amd.com> (raw)
In-Reply-To: <156518035127.6198.2346142882693144294@skylake-alporthouse-com>

Am 07.08.19 um 14:19 schrieb Chris Wilson:
> Quoting Christian König (2019-08-07 13:08:38)
>> Am 06.08.19 um 21:57 schrieb Chris Wilson:
>>> If we add to shared-list during the read, ... Hmm, actually we should
>>> return num_list, i.e.
>>>
>>> do {
>>>        *list = rcu_dereference(obj->fence);
>>>        num_list = *list ? (*list)->count : 0;
>>>        smp_rmb();
>>> } while (...)
>>>
>>> return num_list.
>>>
>>> as the relationship between the count and the fence entries is also
>>> determined by the mb in add_shared_fence.
>> I've read that multiple times now, but can't follow. Why should we do this?
>>
>> The only important thing is that the readers see the new fence before
>> the increment of the number of fences.
> Exactly. We order the store so that the fence is in the list before we
> update the count (so that we don't read garbage because the fence isn't
> there yet).
>
> But we don't have the equivalent here for the read once the rmb is
> removed from the seqcount_read_begin/end looping. We need to see the
> update in the same order as was stored, and only use the coherent
> portion of the list.

Ok that makes sense. Going to fix up the code regarding to that.

Christian.

> -Chris

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