linux-kernel - Re: [PATCH v3 9/9] drm/tegra: Support context isolation

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Date:   Tue, 22 Feb 2022 10:37:05 +0200
From:   Mikko Perttunen <cyndis@...si.fi>
To:     Dmitry Osipenko <digetx@...il.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        thierry.reding@...il.com, jonathanh@...dia.com, joro@...tes.org,
        will@...nel.org, robh+dt@...nel.org, robin.murphy@....com
Cc:     linux-tegra@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 9/9] drm/tegra: Support context isolation

On 2/21/22 22:02, Dmitry Osipenko wrote:
> 21.02.2022 15:06, Mikko Perttunen пишет:
>> On 2/19/22 20:35, Dmitry Osipenko wrote:
>>> 18.02.2022 14:39, Mikko Perttunen пишет:
>>>> +    if (context->memory_context &&
>>>> context->client->ops->get_streamid_offset) {
>>>               ^^^
>>>> +        int offset =
>>>> context->client->ops->get_streamid_offset(context->client);
>>>> +
>>>> +        if (offset >= 0) {
>>>> +            job->context = context->memory_context;
>>>> +            job->engine_streamid_offset = offset;
>>>> +            host1x_context_get(job->context);
>>>> +        }
>>>
>>> You should bump refcount unconditionally or you'll get refcnt underflow
>>> on put, when offset < 0.
>>
>> This refcount is intended to be dropped from 'release_job', where it's
>> dropped if job->context is set, which it is from this path.
>>
>>>
>>>> +    }
>>>> +
>>>>        /*
>>>>         * job_data is now part of job reference counting, so don't
>>>> release
>>>>         * it from here.
>>>> diff --git a/drivers/gpu/drm/tegra/uapi.c b/drivers/gpu/drm/tegra/uapi.c
>>>> index 9ab9179d2026..be33da54d12c 100644
>>>> --- a/drivers/gpu/drm/tegra/uapi.c
>>>> +++ b/drivers/gpu/drm/tegra/uapi.c
>>>> @@ -33,6 +33,9 @@ static void tegra_drm_channel_context_close(struct
>>>> tegra_drm_context *context)
>>>>        struct tegra_drm_mapping *mapping;
>>>>        unsigned long id;
>>>>    +    if (context->memory_context)
>>>> +        host1x_context_put(context->memory_context);
>>>
>>> The "if (context->memory_context &&
>>> context->client->ops->get_streamid_offset)" above doesn't match the "if
>>> (context->memory_context)". You'll get refcount underflow.
>>
>> And this drop is for the refcount implicitly added when allocating the
>> memory context through host1x_context_alloc; so these two places should
>> be independent.
>>
>> Please elaborate if I missed something.
> 
> You named context as memory_context and then have
> context=context->memory_context. Please try to improve the variable
> names, like drm_ctx->host1x_ctx for example.
> 
> I'm also not a big fan of the "if (ref) put(ref)" pattern. Won't be
> better to move all the "if (!NULL)" checks inside of get()/put() and
> make the invocations unconditional?

I agree that the naming here is confusing with different kinds of 
contexts flying around, though I would prefer not to change the original 
'struct tegra_drm_context *context' since it's used all around the code. 
But I'll try to make it clearer.

Regarding moving NULL checks inside get/put, I personally dislike that 
pattern (also with e.g. kfree) since when reading the code, it makes it 
more difficult to see that the pointer can be NULL.

Mikko