From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Date: Fri, 4 Sep 2020 17:32:29 +0200
Subject: seqlock: seqcount_t: Implement all read APIs as statement expressions
Patch-mainline: v5.10-rc1
Git-commit: 52ac39e5db5148f70392edb654ad882ac8da88a8
References: bsc#1176564 bsc#1162702

The sequence counters read APIs are implemented as CPP macros, so they
can take either seqcount_t or any of the seqcount_LOCKNAME_t variants.
Such macros then get *directly* transformed to internal C functions that
only take plain seqcount_t.

Further commits need access to seqcount_LOCKNAME_t inside of the actual
read APIs code. Thus transform all of the seqcount read APIs to pure GCC
statement expressions instead.

This will not break type-safety: all of the transformed APIs resolve to
a _Generic() selection that does not have a "default" case.

This will also not affect the transformed APIs readability: previously
added kernel-doc above all of seqlock.h functions makes the expectations
quite clear for call-site developers.

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/20200904153231.11994-4-a.darwish@linutronix.de
[dwagner: context updated, kcsan bits dropped]
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 include/linux/seqlock.h |   90 ++++++++++++++++++++++--------------------------
 1 file changed, 43 insertions(+), 47 deletions(-)

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -168,6 +168,12 @@ static __always_inline seqcount_t *
 	return &s->seqcount;						\
 }									\
 									\
+static __always_inline unsigned						\
+__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s)	\
+{									\
+	return READ_ONCE(s->seqcount.sequence);				\
+}									\
+									\
 static __always_inline bool						\
 __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s)	\
 {									\
@@ -189,6 +195,11 @@ static inline seqcount_t *__seqprop_ptr(
 	return s;
 }
 
+static inline unsigned __seqprop_sequence(const seqcount_t *s)
+{
+	return READ_ONCE(s->sequence);
+}
+
 static inline bool __seqprop_preemptible(const seqcount_t *s)
 {
 	return false;
@@ -235,6 +246,7 @@ SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mu
 	__seqprop_case((s),	ww_mutex,	prop))
 
 #define __seqcount_ptr(s)		__seqprop(s, ptr)
+#define __seqcount_sequence(s)		__seqprop(s, sequence)
 #define __seqcount_lock_preemptible(s)	__seqprop(s, preemptible)
 #define __seqcount_assert_lock_held(s)	__seqprop(s, assert)
 
@@ -253,20 +265,14 @@ SEQCOUNT_LOCKNAME(ww_mutex,	struct ww_mu
  * Return: count to be passed to read_seqcount_retry()
  */
 #define __read_seqcount_begin(s)					\
-	__read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
-{
-	unsigned ret;
-
-repeat:
-	ret = READ_ONCE(s->sequence);
-	if (unlikely(ret & 1)) {
-		cpu_relax();
-		goto repeat;
-	}
-	return ret;
-}
+({									\
+	unsigned seq;							\
+									\
+	while ((seq = __seqcount_sequence(s)) & 1)			\
+		cpu_relax();						\
+									\
+	seq;								\
+})
 
 /**
  * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
@@ -275,14 +281,12 @@ static inline unsigned __read_seqcount_t
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_read_seqcount_begin(s)					\
-	raw_read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
-{
-	unsigned ret = __read_seqcount_t_begin(s);
-	smp_rmb();
-	return ret;
-}
+({									\
+	unsigned seq = __read_seqcount_begin(s);			\
+									\
+	smp_rmb();							\
+	seq;								\
+})
 
 /**
  * read_seqcount_begin() - begin a seqcount_t read critical section
@@ -291,13 +295,10 @@ static inline unsigned raw_read_seqcount
  * Return: count to be passed to read_seqcount_retry()
  */
 #define read_seqcount_begin(s)						\
-	read_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
-{
-	seqcount_lockdep_reader_access(s);
-	return raw_read_seqcount_t_begin(s);
-}
+({									\
+	seqcount_lockdep_reader_access(__seqcount_ptr(s));		\
+	raw_read_seqcount_begin(s);					\
+})
 
 /**
  * raw_read_seqcount() - read the raw seqcount_t counter value
@@ -311,14 +312,12 @@ static inline unsigned read_seqcount_t_b
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_read_seqcount(s)						\
-	raw_read_seqcount_t(__seqcount_ptr(s))
-
-static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
-{
-	unsigned ret = READ_ONCE(s->sequence);
-	smp_rmb();
-	return ret;
-}
+({									\
+	unsigned seq = __seqcount_sequence(s);				\
+									\
+	smp_rmb();							\
+	seq;								\
+})
 
 /**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
@@ -338,16 +337,13 @@ static inline unsigned raw_read_seqcount
  * Return: count to be passed to read_seqcount_retry()
  */
 #define raw_seqcount_begin(s)						\
-	raw_seqcount_t_begin(__seqcount_ptr(s))
-
-static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
-{
-	/*
-	 * If the counter is odd, let read_seqcount_retry() fail
-	 * by decrementing the counter.
-	 */
-	return raw_read_seqcount_t(s) & ~1;
-}
+({									\
+	/*								\
+	 * If the counter is odd, let read_seqcount_retry() fail	\
+	 * by decrementing the counter.					\
+	 */								\
+	raw_read_seqcount(s) & ~1;					\
+})
 
 /**
  * __read_seqcount_retry() - end a seqcount_t read section w/o barrier