linux-kernel - [PATCH 04/21] context_tracking: Add a note about noinstr VS unsafe context tracking functions

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Message-Id: <20220503100051.2799723-5-frederic@kernel.org>
Date:   Tue,  3 May 2022 12:00:34 +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 04/21] context_tracking: Add a note about noinstr VS unsafe context tracking functions

Some context tracking 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 functions can't get
tagged as noinstr even though they should.

Reported-by: Peter Zijlstra <peterz@...radead.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>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/context_tracking.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 36a98c48aedc..1f44b0461d14 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -103,6 +103,11 @@ void noinstr __context_tracking_enter(enum ctx_state state)
 }
 EXPORT_SYMBOL_GPL(__context_tracking_enter);
 
+/*
+ * 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 context_tracking_enter(enum ctx_state state)
 {
 	unsigned long flags;
@@ -125,6 +130,10 @@ void context_tracking_enter(enum ctx_state state)
 NOKPROBE_SYMBOL(context_tracking_enter);
 EXPORT_SYMBOL_GPL(context_tracking_enter);
 
+/*
+ * FIXME: This function should be noinstr but it unsafely calls local_irq_restore(),
+ * involving illegal RCU uses through tracing and lockdep. This must be fixed first.
+ */
 void context_tracking_user_enter(void)
 {
 	user_enter();
@@ -168,6 +177,11 @@ void noinstr __context_tracking_exit(enum ctx_state state)
 }
 EXPORT_SYMBOL_GPL(__context_tracking_exit);
 
+/*
+ * 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 context_tracking_exit(enum ctx_state state)
 {
 	unsigned long flags;
@@ -182,6 +196,10 @@ void context_tracking_exit(enum ctx_state state)
 NOKPROBE_SYMBOL(context_tracking_exit);
 EXPORT_SYMBOL_GPL(context_tracking_exit);
 
+/*
+ * FIXME: This function should be noinstr but it unsafely calls local_irq_save(),
+ * involving illegal RCU uses through tracing and lockdep. This must be fixed first.
+ */
 void context_tracking_user_exit(void)
 {
 	user_exit();
-- 
2.25.1