drm/i915/gem: Serialise debugfs i915_gem_objects with ctx->mutex

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
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200723172119.17649-3-chris@chris-wilson.co.uk
(cherry picked from commit 102f5aa491f262c818e607fc4fee08a724a76c69)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 784219962193785e93b943532973cdc50aac1d95..ea469168cd44326b02198c2b1b087018b39d9371 100644 (file)
--- 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);