linux-kernel - [RFC 04/17] drm/cgroup: Allow safe external access to file

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Date:   Wed, 19 Oct 2022 18:32:41 +0100
From:   Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:     Intel-gfx@...ts.freedesktop.org
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Dave Airlie <airlied@...hat.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Rob Clark <robdclark@...omium.org>,
        Stéphane Marchesin <marcheu@...omium.org>,
        "T . J . Mercier" <tjmercier@...gle.com>, Kenny.Ho@....com,
        Christian König <christian.koenig@....com>,
        Brian Welty <brian.welty@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>
Subject: [RFC 04/17] drm/cgroup: Allow safe external access to file_priv

From: Tvrtko Ursulin <tvrtko.ursulin@...el.com>

Entry points from the cgroup subsystem into the drm cgroup controller will
need to walk the file_priv structures associated with registered clients
and since those are not RCU protected lets add a hack for now to make this
safe.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@...el.com>
---
 drivers/gpu/drm/drm_cgroup.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/drm_cgroup.c b/drivers/gpu/drm/drm_cgroup.c
index a31ff1d593ab..9e9caeb0aa87 100644
--- a/drivers/gpu/drm/drm_cgroup.c
+++ b/drivers/gpu/drm/drm_cgroup.c
@@ -21,6 +21,13 @@ void drm_clients_close(struct drm_file *file_priv)
 	if (atomic_dec_and_test(&clients->num)) {
 		xa_erase(&drm_pid_clients, pid);
 		kfree_rcu(clients, rcu);
+
+		/*
+		 * FIXME: file_priv is not RCU protected so we add this hack
+		 * to avoid any races with code which walks clients->file_list
+		 * and accesses file_priv.
+		 */
+		synchronize_rcu();
 	}
 }
 
-- 
2.34.1