From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jesse Barnes <jbarnes@virtuousgeek.org>
Subject: Re: [PATCH 01/12] drm/i915: Assert mutex_is_locked on
 context lookup
Date: Thu, 2 May 2013 13:27:32 -0700
Message-ID: <20130502132732.169fb0ff@jbarnes-desktop>
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To: Ben Widawsky <ben@bwidawsk.net>
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On Tue, 23 Apr 2013 23:15:29 -0700
Ben Widawsky <ben@bwidawsk.net> wrote:

> Because our context refcounting doesn't grab a ref at lookup time, it is
> unsafe to do so without the lock.
> 
> NOTE: We don't have an easy way to put the assertion in the lookup
> function which is where this really belongs. Context switching is good
> enough because it actually asserts even more correctness by protecting
> the default_context.
> 
> Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> ---
>  drivers/gpu/drm/i915/i915_gem_context.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index a1e8ecb..411ace0 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -444,6 +444,8 @@ int i915_switch_context(struct intel_ring_buffer *ring,
>  	if (dev_priv->hw_contexts_disabled)
>  		return 0;
>  
> +	BUG_ON(!mutex_is_locked(&dev_priv->dev->struct_mutex));
> +
>  	if (ring != &dev_priv->ring[RCS])
>  		return 0;
>  

Simple enough.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>

We usually do WARN_ONs for this stuff though, in case a user actually
does hit it, it may not be fatal so why crash the machine?

But that's a minor distinction since we shouldn't hit this except in
development anyway.

-- 
Jesse Barnes, Intel Open Source Technology Center