[PATCH 5.17 070/772] rcu-tasks: Handle sparse cpu_possible_mask in rcu_tasks_invoke_cbs()

Greg Kroah-Hartman
2022-06-07T20:28:36Z

From: Paul E. McKenney <paulmck@kernel.org>

[ Upstream commit ab2756ea6b74987849b44ad0e33c3cfec159033b ]

If the cpu_possible_mask is sparse (for example, if bits are set only for
CPUs 0, 4, 8, ...), then rcu_tasks_invoke_cbs() will access per-CPU data
for a CPU not in cpu_possible_mask. It makes these accesses while doing
a workqueue-based binary search for non-empty callback lists. Although
this search must pass through CPUs not represented in cpu_possible_mask,
it has no need to check the callback list for such CPUs.

This commit therefore changes the rcu_tasks_invoke_cbs() function's
binary search so as to only check callback lists for CPUs present in
cpu_possible_mask.

Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/rcu/tasks.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 30c42797f53b..1664e472524b 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -460,7 +460,7 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
}
}

- if (rcu_segcblist_empty(&rtpcp->cblist))
+ if (rcu_segcblist_empty(&rtpcp->cblist) || !cpu_possible(cpu))
return;
raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
--
2.35.1