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From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     linux-kernel@vger.kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <vschneid@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: [PATCH 2/9] preempt: Provide preempt_[dis|en]able_nested()
Date:   Wed, 17 Aug 2022 18:26:56 +0200
Message-Id: <20220817162703.728679-3-bigeasy@linutronix.de>
In-Reply-To: <20220817162703.728679-1-bigeasy@linutronix.de>
References: <20220817162703.728679-1-bigeasy@linutronix.de>
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From: Thomas Gleixner <tglx@linutronix.de>

On PREEMPT_RT enabled kernels, spinlocks and rwlocks are neither disabling
preemption nor interrupts. Though there are a few places which depend on
the implicit preemption/interrupt disable of those locks, e.g. seqcount
write sections, per CPU statistics updates etc.

To avoid sprinkling CONFIG_PREEMPT_RT conditionals all over the place, add
preempt_disable_nested() and preempt_enable_nested() which should be
descriptive enough.

Add a lockdep assertion for the !PREEMPT_RT case to catch callers which
do not have preemption disabled.

Cc: Ben Segall <bsegall@google.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/preempt.h | 42 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index b4381f255a5ca..0df425bf9bd75 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -421,4 +421,46 @@ static inline void migrate_enable(void) { }
=20
 #endif /* CONFIG_SMP */
=20
+/**
+ * preempt_disable_nested - Disable preemption inside a normally preempt d=
isabled section
+ *
+ * Use for code which requires preemption protection inside a critical
+ * section which has preemption disabled implicitly on non-PREEMPT_RT
+ * enabled kernels, by e.g.:
+ *  - holding a spinlock/rwlock
+ *  - soft interrupt context
+ *  - regular interrupt handlers
+ *
+ * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft
+ * interrupt context and regular interrupt handlers are preemptible and
+ * only prevent migration. preempt_disable_nested() ensures that preemption
+ * is disabled for cases which require CPU local serialization even on
+ * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP.
+ *
+ * The use cases are code sequences which are not serialized by a
+ * particular lock instance, e.g.:
+ *  - seqcount write side critical sections where the seqcount is not
+ *    associated to a particular lock and therefore the automatic
+ *    protection mechanism does not work. This prevents a live lock
+ *    against a preempting high priority reader.
+ *  - RMW per CPU variable updates like vmstat.
+ */
+/* Macro to avoid header recursion hell vs. lockdep */
+#define preempt_disable_nested()				\
+do {								\
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))			\
+		preempt_disable();				\
+	else							\
+		lockdep_assert_preemption_disabled();		\
+} while (0)
+
+/**
+ * preempt_enable_nested - Undo the effect of preempt_disable_nested()
+ */
+static __always_inline void preempt_enable_nested(void)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
+}
+
 #endif /* __LINUX_PREEMPT_H */
--=20
2.37.2