linux-kernel - [PATCH 03/21] rcu: Add a note about noinstr VS unsafe eqs functions

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Date:   Tue,  3 May 2022 12:00:33 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu Liao <liaoyu15@...wei.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 03/21] rcu: Add a note about noinstr VS unsafe eqs functions

Some RCU functions enter or exit into/from RCU idle mode while using
trace-able and lockdep-aware IRQs (un-)masking. This could be easily
solved with using raw versions of local_irq_*() but we would then
lose some precious debugging informations.

Another possible way to solve this may consist in using rude RCU-tasks
in lockdep and irqsoff tracing.

In any case and until this get solved, those RCU functions can't get
tagged as noinstr even though they should.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Nicolas Saenz Julienne <nsaenz@...nel.org>
Cc: Marcelo Tosatti <mtosatti@...hat.com>
Cc: Xiongfeng Wang <wangxiongfeng2@...wei.com>
Cc: Yu Liao<liaoyu15@...wei.com>
Cc: Phil Auld <pauld@...hat.com>
Cc: Paul Gortmaker<paul.gortmaker@...driver.com>
Cc: Alex Belits <abelits@...vell.com>
---
 kernel/rcu/tree.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a5da4b87afde..2a6e7723fc4d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -838,6 +838,10 @@ void rcu_irq_exit_check_preempt(void)
  *
  * If you add or remove a call to rcu_irq_exit_irqson(), be sure to test
  * with CONFIG_RCU_EQS_DEBUG=y.
+ *
+ * FIXME: This function should be noinstr but the below local_irq_restore() is
+ * unsafe because it involves illegal RCU uses through tracing and lockdep.
+ * This must be fixed first.
  */
 void rcu_irq_exit_irqson(void)
 {
@@ -895,6 +899,10 @@ static void noinstr rcu_eqs_exit(bool user)
  *
  * If you add or remove a call to rcu_idle_exit(), be sure to test with
  * CONFIG_RCU_EQS_DEBUG=y.
+ *
+ * FIXME: This function should be noinstr but the below local_irq_save() is
+ * unsafe because it involves illegal RCU uses through tracing and lockdep.
+ * This must be fixed first.
  */
 void rcu_idle_exit(void)
 {
@@ -1076,6 +1084,10 @@ noinstr void rcu_irq_enter(void)
  *
  * If you add or remove a call to rcu_irq_enter_irqson(), be sure to test
  * with CONFIG_RCU_EQS_DEBUG=y.
+ *
+ * FIXME: This function should be noinstr but the below local_irq_save() is
+ * unsafe because it involves illegal RCU uses through tracing and lockdep.
+ * This must be fixed first.
  */
 void rcu_irq_enter_irqson(void)
 {
-- 
2.25.1