From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Date: Mon, 20 Jul 2020 17:55:14 +0200
Subject: seqlock: lockdep assert non-preemptibility on seqcount_t write
Patch-mainline: v5.9-rc1
Git-commit: 859247d39fb008ea812e8f0c398a58a20c12899e
References: bsc#1176564 bsc#1162702

Preemption must be disabled before entering a sequence count write side
critical section.  Failing to do so, the seqcount read side can preempt
the write side section and spin for the entire scheduler tick.  If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.

Assert through lockdep that preemption is disabled for seqcount writers.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200720155530.1173732-9-a.darwish@linutronix.de
[dwagner: update context]
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 include/linux/seqlock.h |   29 +++++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -417,6 +417,12 @@ static inline void raw_write_seqcount_la
        smp_wmb();      /* increment "sequence" before following stores */
 }
 
+static inline void __write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+	raw_write_seqcount_begin(s);
+	seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
 /**
  * write_seqcount_begin_nested() - start a seqcount_t write section with
  *                                 custom lockdep nesting level
@@ -427,8 +433,19 @@ static inline void raw_write_seqcount_la
  */
 static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
 {
-	raw_write_seqcount_begin(s);
-	seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+	lockdep_assert_preemption_disabled();
+	__write_seqcount_begin_nested(s, subclass);
+}
+
+/*
+ * A write_seqcount_begin() variant w/o lockdep non-preemptibility checks.
+ *
+ * Use for internal seqlock.h code where it's known that preemption is
+ * already disabled. For example, seqlock_t write side functions.
+ */
+static inline void __write_seqcount_begin(seqcount_t *s)
+{
+	__write_seqcount_begin_nested(s, 0);
 }
 
 /**
@@ -540,7 +557,7 @@ static inline unsigned read_seqretry(con
 static inline void write_seqlock(seqlock_t *sl)
 {
 	spin_lock(&sl->lock);
-	write_seqcount_begin(&sl->seqcount);
+	__write_seqcount_begin(&sl->seqcount);
 }
 
 /**
@@ -566,7 +583,7 @@ static inline void write_sequnlock(seqlo
 static inline void write_seqlock_bh(seqlock_t *sl)
 {
 	spin_lock_bh(&sl->lock);
-	write_seqcount_begin(&sl->seqcount);
+	__write_seqcount_begin(&sl->seqcount);
 }
 
 /**
@@ -593,7 +610,7 @@ static inline void write_sequnlock_bh(se
 static inline void write_seqlock_irq(seqlock_t *sl)
 {
 	spin_lock_irq(&sl->lock);
-	write_seqcount_begin(&sl->seqcount);
+	__write_seqcount_begin(&sl->seqcount);
 }
 
 /**
@@ -614,7 +631,7 @@ static inline unsigned long __write_seql
 	unsigned long flags;
 
 	spin_lock_irqsave(&sl->lock, flags);
-	write_seqcount_begin(&sl->seqcount);
+	__write_seqcount_begin(&sl->seqcount);
 	return flags;
 }