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

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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



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