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

[Daniel Vetter]

On Mon, Sep 14, 2020 at 05:45:09PM +0100, Tvrtko Ursulin wrote:
> 
> 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 at chris-wilson.co.uk>
> > Cc: CQ Tang <cq.tang at intel.com>
> > Cc: Daniel Vetter <daniel.vetter at intel.com>
> > Cc: stable at 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 at intel.com>

I'm not super thrilled about patch 1 in this, for debugfs imo better to
wrangle this in the driver. And without patch 1 and 2 this wont fix a
whole lot.
-Daniel


> 
> Regards,
> 
> Tvrtko
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch