[Mesa-dev] [PATCH 1/6] st/mesa: fix sampler view context mismatch issue

[Brian Paul]

On Sun, Mar 10, 2019 at 1:20 AM Mathias Fröhlich <Mathias.Froehlich at gmx.net>
wrote:

> Brian,
>
> One question also for my own education inline below:
>
> On Friday, 8 March 2019 23:52:09 CET Brian Paul wrote:
> > After a while of running google-chrome and viewing Bing maps, we'd see
> > "context mismatch in svga_sampler_view_destroy()" messages and
> > eventually crash because we leaked too many sampler views (the kernel
> > module would have too many sampler views).
> >
> > When a texture object is being deleted, we call
> > st_texture_release_all_sampler_views() to release all the sampler
> > views.  In the list, there may sampler views which were created from
> > other contexts.
> >
> > Previously, we called pipe_sampler_view_release(pipe, view) which would
> > use the given pipe context to destroy the view if the refcount hit
> > zero.  The svga error was triggered because we were calling
> > pipe->sampler_view_destroy(pipe, view) and the pipe context didn't
> > match the view's parent context.
> >
> > Instead, call pipe_sampler_reference(&view, NULL).  That way, if
> > the refcount hits zero, we'll use the view's parent context to
> > destroy the view.  That's what we want.
>
> That sounds plausible so far.
>
> What I wonder, is calling into an 'arbitrary' different context and do
> work there
> thread safe for any gallium driver?
>

Right.  Before this patch there was the problem of trying to destroy a
sampler view with a context different from the one which created it.
That's fixed, but now there's the multi-threading issue as you say, and
which I believe, is a less-common situation.



>
> Especially since pipe_sampler_view_reference in its documentation says
> that the
> views provided need to originate from the same context and that this must
> be the 'current'.
>
> I have been taking a quick look into iris and radeonsi and both seem to be
> safe
> in terms of threads for dereferencing the views finally.
> But either the documentation of pipe_sampler_view_reference should be
> updated
> or I may miss an other piece of the puzzle to see that it is still save to
> do so.
>

Last week I posted a patch series for the VMware driver which handled the
thread issue with a list of "zombie" sampler views.  Jose suggested moving
that up into the state tracker, and that is a better long-term solution.

I'm almost done with a patch series that does that.  I'll likely post it
tomorrow.

-Brian
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