[PATCH 23/48] rcu: Remove redundant READ_ONCE() of rcu_state.gp_flags in tree.c

Uladzislau Rezki (Sony)
2024-05-07T09:35:56Z

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

Although it is functionally OK to do READ_ONCE() of a variable that
cannot change, it is confusing and at best an accident waiting to happen.
This commit therefore removes a number of READ_ONCE(rcu_state.gp_flags)
instances from kernel/rcu/tree.c that are not needed due to updates
to this field being excluded by virtue of holding the root rcu_node
structure's ->lock.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/lkml/4857c5ef-bd8f-4670-87ac-0600a1699d05@paulmck-laptop/T/#mccb23c2a4902da4d3c750165329f8de056903c58
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Closes: https://lore.kernel.org/lkml/4857c5ef-bd8f-4670-87ac-0600a1699d05@paulmck-laptop/T/#md1b5c026584f9c3c7b0fbc9240dd7de584597b73
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
kernel/rcu/tree.c | 11 ++++-------
1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2795a1457acf..559f2d0d271f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1463,7 +1463,7 @@ static noinline_for_stack bool rcu_gp_init(void)

WRITE_ONCE(rcu_state.gp_activity, jiffies);
raw_spin_lock_irq_rcu_node(rnp);
- if (!READ_ONCE(rcu_state.gp_flags)) {
+ if (!rcu_state.gp_flags) {
/* Spurious wakeup, tell caller to go back to sleep. */
raw_spin_unlock_irq_rcu_node(rnp);
return false;
@@ -1648,8 +1648,7 @@ static void rcu_gp_fqs(bool first_time)
/* Clear flag to prevent immediate re-entry. */
if (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) {
raw_spin_lock_irq_rcu_node(rnp);
- WRITE_ONCE(rcu_state.gp_flags,
- READ_ONCE(rcu_state.gp_flags) & ~RCU_GP_FLAG_FQS);
+ WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags & ~RCU_GP_FLAG_FQS);
raw_spin_unlock_irq_rcu_node(rnp);
}
}
@@ -1910,8 +1909,7 @@ static void rcu_report_qs_rsp(unsigned long flags)
{
raw_lockdep_assert_held_rcu_node(rcu_get_root());
WARN_ON_ONCE(!rcu_gp_in_progress());
- WRITE_ONCE(rcu_state.gp_flags,
- READ_ONCE(rcu_state.gp_flags) | RCU_GP_FLAG_FQS);
+ WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_FQS);
raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(), flags);
rcu_gp_kthread_wake();
}
@@ -2426,8 +2424,7 @@ void rcu_force_quiescent_state(void)
raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
return; /* Someone beat us to it. */
}
- WRITE_ONCE(rcu_state.gp_flags,
- READ_ONCE(rcu_state.gp_flags) | RCU_GP_FLAG_FQS);
+ WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_FQS);
raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
rcu_gp_kthread_wake();
}
--
2.39.2