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

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Date:   Mon, 21 Feb 2022 23:02:28 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Mikko Perttunen <cyndis@...si.fi>,
        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

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?