Re: [Intel-gfx] [PATCH 3/3] drm/i915/gem: Serialise debugfs i915_gem_objects with ctx->mutex

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 23/07/2020 18:21, Chris Wilson wrote:
> Since the debugfs may peek into the GEM contexts as the corresponding
> client/fd is being closed, we may try and follow a dangling pointer.
> However, the context closure itself is serialised with the ctx->mutex,
> so if we hold that mutex as we inspect the state coupled in the context,
> we know the pointers within the context are stable and will remain valid
> as we inspect their tables.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: CQ Tang <cq.tang@intel.com>
> Cc: Daniel Vetter <daniel.vetter@intel.com>
> Cc: stable@vger.kernel.org
> ---
>   drivers/gpu/drm/i915/i915_debugfs.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 784219962193..ea469168cd44 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -326,6 +326,7 @@ static void print_context_stats(struct seq_file *m,
>   		}
>   		i915_gem_context_unlock_engines(ctx);
>   
> +		mutex_lock(&ctx->mutex);
>   		if (!IS_ERR_OR_NULL(ctx->file_priv)) {
>   			struct file_stats stats = {
>   				.vm = rcu_access_pointer(ctx->vm),
> @@ -346,6 +347,7 @@ static void print_context_stats(struct seq_file *m,
>   
>   			print_file_stats(m, name, stats);
>   		}
> +		mutex_unlock(&ctx->mutex);
>   
>   		spin_lock(&i915->gem.contexts.lock);
>   		list_safe_reset_next(ctx, cn, link);
> 

Hm this apparently never got it's r-b and so got re-discovered in the 
field. +Nikunj

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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