[PATCH 6/6] rcu/exp: Remove redundant full memory barrier at the end of GP

Frederic Weisbecker
2024-05-15T12:53:52Z

A full memory barrier is necessary at the end of the expedited grace
period to order:

1) The grace period completion (pictured by the GP sequence
number) with all preceding accesses. This pairs with rcu_seq_end()
performed by the concurrent kworker.

2) The grace period completion and subsequent post-GP update side
accesses. Pairs again against rcu_seq_end().

This full barrier is already provided by the final sync_exp_work_done()
test, making the subsequent explicit one redundant. Remove it and
improve comments.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/tree_exp.h | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index bec24ea6777e..721cb93b1fec 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -265,7 +265,12 @@ static bool sync_exp_work_done(unsigned long s)
{
if (rcu_exp_gp_seq_done(s)) {
trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
- smp_mb(); /* Ensure test happens before caller kfree(). */
+ /*
+ * Order GP completion with preceding accesses. Order also GP
+ * completion with post GP update side accesses. Pairs with
+ * rcu_seq_end().
+ */
+ smp_mb();
return true;
}
return false;
@@ -959,7 +964,6 @@ void synchronize_rcu_expedited(void)
rnp = rcu_get_root();
wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
sync_exp_work_done(s));
- smp_mb(); /* Work actions happen before return. */

/* Let the next expedited grace period start. */
mutex_unlock(&rcu_state.exp_mutex);
--
2.44.0