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Subject[tip: core/rcu] rcu/nocb: Allow de-offloading rdp leader
The following commit has been merged into the core/rcu branch of tip:

Commit-ID: 552cac80e65f38a0cc9022456c09efed7e88f9d6
Gitweb: https://git.kernel.org/tip/552cac80e65f38a0cc9022456c09efed7e88f9d6
Author: Frederic Weisbecker <frederic@kernel.org>
AuthorDate: Tue, 23 Feb 2021 01:10:06 +01:00
Committer: Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Wed, 12 May 2021 12:10:23 -07:00

rcu/nocb: Allow de-offloading rdp leader

The only thing that prevented an rdp leader from being de-offloaded was
the nocb_bypass_timer that used to lock the nocb_lock of the rdp leader.

If an rdp gets de-offloaded, it will subtlely ignore rcu_nocb_lock()
calls and do its job in the timer unsafely. Worse yet: If it gets
re-offloaded in the middle of the timer, rcu_nocb_unlock() would try to
unlock, leaving it imbalanced.

Now that the nocb_bypass_timer doesn't use the nocb_lock anymore,
de-offloading the rdp leader is now safe. This commit therefore allows
the rdp leader to be de-offloaded.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree_plugin.h | 4 ----
1 file changed, 4 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 82e9ffb..015adec 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2489,10 +2489,6 @@ int rcu_nocb_cpu_deoffload(int cpu)
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
int ret = 0;

- if (rdp == rdp->nocb_gp_rdp) {
- pr_info("Can't deoffload an rdp GP leader (yet)\n");
- return -EINVAL;
- }
mutex_lock(&rcu_state.barrier_mutex);
cpus_read_lock();
if (rcu_rdp_is_offloaded(rdp)) {
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