[PATCH] sched_clock: Disable seqlock lockdep usage in sched_clock

[John Stultz]

Unforunately the seqlock lockdep enablmenet can't be used
in sched_clock, since the lockdep infrastructure eventually
calls into sched_clock, which causes a deadlock.

Thus, this patch adds _no_lockdep() seqlock methods for the
writer side, and changes all generic sched_clock usage to use
the _no_lockdep methods.

This solves the issue I was able to reproduce, but it would
be good to get Krzysztof to confirm it solves his problem.

Cc: Krzysztof Hałasa <khalasa at piap.pl>
Cc: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
Cc: Willy Tarreau <w at 1wt.eu>
Cc: Ingo Molnar <mingo at kernel.org>,
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Stephen Boyd <sboyd at codeaurora.org>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: linux-arm-kernel at lists.infradead.org
Reported-by: Krzysztof Hałasa <khalasa at piap.pl>
Signed-off-by: John Stultz <john.stultz at linaro.org>
---
 include/linux/seqlock.h   | 19 +++++++++++++++----
 kernel/time/sched_clock.c |  6 +++---
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index cf87a24..7664f68 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -206,14 +206,26 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
 }
 
 
+
+static inline void write_seqcount_begin_no_lockdep(seqcount_t *s)
+{
+	s->sequence++;
+	smp_wmb();
+}
+
+static inline void write_seqcount_end_no_lockdep(seqcount_t *s)
+{
+	smp_wmb();
+	s->sequence++;
+}
+
 /*
  * Sequence counter only version assumes that callers are using their
  * own mutexing.
  */
 static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
 {
-	s->sequence++;
-	smp_wmb();
+	write_seqcount_begin_no_lockdep(s);
 	seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
 }
 
@@ -225,8 +237,7 @@ static inline void write_seqcount_begin(seqcount_t *s)
 static inline void write_seqcount_end(seqcount_t *s)
 {
 	seqcount_release(&s->dep_map, 1, _RET_IP_);
-	smp_wmb();
-	s->sequence++;
+	write_seqcount_end_no_lockdep(s);
 }
 
 /**
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 68b7993..13561a0 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -74,7 +74,7 @@ unsigned long long notrace sched_clock(void)
 		return cd.epoch_ns;
 
 	do {
-		seq = read_seqcount_begin(&cd.seq);
+		seq = read_seqcount_begin_no_lockdep(&cd.seq);
 		epoch_cyc = cd.epoch_cyc;
 		epoch_ns = cd.epoch_ns;
 	} while (read_seqcount_retry(&cd.seq, seq));
@@ -99,10 +99,10 @@ static void notrace update_sched_clock(void)
 			  cd.mult, cd.shift);
 
 	raw_local_irq_save(flags);
-	write_seqcount_begin(&cd.seq);
+	write_seqcount_begin_no_lockdep(&cd.seq);
 	cd.epoch_ns = ns;
 	cd.epoch_cyc = cyc;
-	write_seqcount_end(&cd.seq);
+	write_seqcount_end_no_lockdep(&cd.seq);
 	raw_local_irq_restore(flags);
 }
 
-- 
1.8.3.2