Message-ID: <a69c7362-52a5-ed26-ce12-69364d12fcf6@redhat.com>
Date:   Tue, 7 Feb 2023 11:43:39 +0100
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Subject: Re: [PATCH] drm/shmem-helper: Fix locking for
 drm_gem_shmem_get_pages_sgt()
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To:     Asahi Lina <lina@asahilina.net>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>
Cc:     Alyssa Rosenzweig <alyssa@rosenzweig.io>,
        =?UTF-8?Q?Noralf_Tr=c3=b8nnes?= <noralf@tronnes.org>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        asahi@lists.linux.dev
References: <20230205125124.2260-1-lina@asahilina.net>
 <a1cd8c74-98f4-bff0-0344-cbece787c6e8@redhat.com>
 <d3331cf8-02df-bf15-586b-af9d10830758@asahilina.net>
From:   Javier Martinez Canillas <javierm@redhat.com>
In-Reply-To: <d3331cf8-02df-bf15-586b-af9d10830758@asahilina.net>
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On 2/7/23 11:33, Asahi Lina wrote:
> On 07/02/2023 03.47, Javier Martinez Canillas wrote:
>> Hello Lina,
>>
>> On 2/5/23 13:51, Asahi Lina wrote:
>>> Other functions touching shmem->sgt take the pages lock, so do that here
>>> too. drm_gem_shmem_get_pages() & co take the same lock, so move to the
>>> _locked() variants to avoid recursive locking.
>>>
>>> Discovered while auditing locking to write the Rust abstractions.
>>>
>>> Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
>>> Fixes: 4fa3d66f132b ("drm/shmem: Do dma_unmap_sg before purging pages")
>>> Signed-off-by: Asahi Lina <lina@asahilina.net>
>>> ---
>>
>> Good catch. The patch looks good to me.
>>
>> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
>>
>> What about drm_gem_shmem_free() BTW, I believe that the helper should also
>> grab the lock before unmap / free the sgtable?
> 
> That's called from driver free callbacks, so it should only be called
> when there are no other users left and the refcount is zero, right? If
> there's anyone else racing it I think we have bigger problems than the
> pages lock at that point, since the last thing it does is `kfree(shmem);` ^^
>

Yes, I was wondering only for the critical section that does:

		if (shmem->sgt) {
			dma_unmap_sgtable(obj->dev->dev, shmem->sgt,
					  DMA_BIDIRECTIONAL, 0);
			sg_free_table(shmem->sgt);
			kfree(shmem->sgt);
		}
		if (shmem->pages)
			drm_gem_shmem_put_pages(shmem);
 
> (In Rust terms this is equivalent to the Drop trait, which takes a
> mutable/exclusive reference, which means no other reference to the
> object can exist at that point, so no races are possible. And in fact in
> my Rust abstraction I trigger a drop of the Rust object embedded in the
> shmem object before calling drm_gem_shmem_free(), so if this invariant
> doesn't hold that code would be wrong too!)
>

But I guess you are correct and would be safe to assume that the .free
callback won't race against other struct drm_gem_object_funcs handlers.
I just felt to ask since wasn't sure about that.
 
I'll wait a few days in case someone else wants to take a look to your
patch and then push it to drm-misc. Thanks again! 

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat