github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-05-29 17:43:52 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

rcu/exp: Remove confusing needless full barrier on task unblock

Browse Source 
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>
This commit is contained in:
Frederic Weisbecker 2025-04-29 15:43:01 +02:00 committed by Neeraj Upadhyay (AMD)
parent f2e555fc04
commit 4b9432ed65
1 changed files with 0 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
kernel/rcu/tree_plugin.h
View File

@ -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;