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

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Message-ID: <ea65fcd6-9451-7f5b-f4a9-57b8575f09ff@kapsi.fi>
Date:   Mon, 21 Feb 2022 14:06:43 +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 2/19/22 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.

Thanks,
Mikko