diff options
| author | Frederic Weisbecker <frederic@kernel.org> | 2025-04-29 15:43:01 +0200 |
|---|---|---|
| committer | Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org> | 2025-07-08 23:20:19 +0530 |
| commit | 4b9432ed65cb3a692e8d8bd86abb93f5f2dc5b69 (patch) | |
| tree | a3ea1e00917f54b378b523b576b359afb3f79a09 | |
| parent | f2e555fc04ba238cd1dd21040325fa0629ee433d (diff) | |
rcu/exp: Remove confusing needless full barrier on task unblock
A full memory barrier in the RCU-PREEMPT task unblock path advertizes
to order the context switch (or rather the accesses prior to
rcu_read_unlock()) with the expedited grace period fastpath.
However the grace period can not complete without the rnp calling into
rcu_report_exp_rnp() with the node locked. This reports the quiescent
state in a fully ordered fashion against updater's accesses thanks to:
1) The READ-SIDE smp_mb__after_unlock_lock() barrier across nodes
locking while propagating QS up to the root.
2) The UPDATE-SIDE smp_mb__after_unlock_lock() barrier while holding the
the root rnp to wait/check for the GP completion.
3) The (perhaps redundant given step 1) and 2)) smp_mb() in rcu_seq_end()
before the grace period completes.
This makes the explicit barrier in this place superfluous. Therefore
remove it as it is confusing.
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
| -rw-r--r-- | kernel/rcu/tree_plugin.h | 1 |
1 files changed, 0 insertions, 1 deletions
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 0b0f56f6abc8..0532a13cb75e 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -534,7 +534,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq && (!empty_norm || rnp->qsmask)); empty_exp = sync_rcu_exp_done(rnp); - smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */ np = rcu_next_node_entry(t, rnp); list_del_init(&t->rcu_node_entry); t->rcu_blocked_node = NULL; |