1 files changed, 96 insertions, 27 deletions

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index fffeb754880f..5ce48eab7a2a 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -272,7 +272,7 @@ SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     mutex)
 ({									\
 	unsigned __seq;							\
 									\
-	while ((__seq = seqprop_sequence(s)) & 1)			\
+	while (unlikely((__seq = seqprop_sequence(s)) & 1))		\
 		cpu_relax();						\
 									\
 	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
@@ -319,6 +319,29 @@ SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     mutex)
 })
 
 /**
+ * raw_seqcount_try_begin() - begin a seqcount_t read critical section
+ *                            w/o lockdep and w/o counter stabilization
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
+ * @start: count to be passed to read_seqcount_retry()
+ *
+ * Similar to raw_seqcount_begin(), except it enables eliding the critical
+ * section entirely if odd, instead of doing the speculation knowing it will
+ * fail.
+ *
+ * Useful when counter stabilization is more or less equivalent to taking
+ * the lock and there is a slowpath that does that.
+ *
+ * If true, start will be set to the (even) sequence count read.
+ *
+ * Return: true when a read critical section is started.
+ */
+#define raw_seqcount_try_begin(s, start)				\
+({									\
+	start = raw_read_seqcount(s);					\
+	!(start & 1);							\
+})
+
+/**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
  *                        lockdep and w/o counter stabilization
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
@@ -622,6 +645,23 @@ static __always_inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *
 }
 
 /**
+ * read_seqcount_latch() - pick even/odd latch data copy
+ * @s: Pointer to seqcount_latch_t
+ *
+ * See write_seqcount_latch() for details and a full reader/writer usage
+ * example.
+ *
+ * Return: sequence counter raw value. Use the lowest bit as an index for
+ * picking which data copy to read. The full counter must then be checked
+ * with read_seqcount_latch_retry().
+ */
+static __always_inline unsigned read_seqcount_latch(const seqcount_latch_t *s)
+{
+	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
+	return raw_read_seqcount_latch(s);
+}
+
+/**
  * raw_read_seqcount_latch_retry() - end a seqcount_latch_t read section
  * @s:		Pointer to seqcount_latch_t
  * @start:	count, from raw_read_seqcount_latch()
@@ -636,8 +676,33 @@ raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
 }
 
 /**
+ * read_seqcount_latch_retry() - end a seqcount_latch_t read section
+ * @s:		Pointer to seqcount_latch_t
+ * @start:	count, from read_seqcount_latch()
+ *
+ * Return: true if a read section retry is required, else false
+ */
+static __always_inline int
+read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
+{
+	kcsan_atomic_next(0);
+	return raw_read_seqcount_latch_retry(s, start);
+}
+
+/**
  * raw_write_seqcount_latch() - redirect latch readers to even/odd copy
  * @s: Pointer to seqcount_latch_t
+ */
+static __always_inline void raw_write_seqcount_latch(seqcount_latch_t *s)
+{
+	smp_wmb();	/* prior stores before incrementing "sequence" */
+	s->seqcount.sequence++;
+	smp_wmb();      /* increment "sequence" before following stores */
+}
+
+/**
+ * write_seqcount_latch_begin() - redirect latch readers to odd copy
+ * @s: Pointer to seqcount_latch_t
  *
  * The latch technique is a multiversion concurrency control method that allows
  * queries during non-atomic modifications. If you can guarantee queries never
@@ -665,17 +730,11 @@ raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
  *
  *	void latch_modify(struct latch_struct *latch, ...)
  *	{
- *		smp_wmb();	// Ensure that the last data[1] update is visible
- *		latch->seq.sequence++;
- *		smp_wmb();	// Ensure that the seqcount update is visible
- *
+ *		write_seqcount_latch_begin(&latch->seq);
  *		modify(latch->data[0], ...);
- *
- *		smp_wmb();	// Ensure that the data[0] update is visible
- *		latch->seq.sequence++;
- *		smp_wmb();	// Ensure that the seqcount update is visible
- *
+ *		write_seqcount_latch(&latch->seq);
  *		modify(latch->data[1], ...);
+ *		write_seqcount_latch_end(&latch->seq);
  *	}
  *
  * The query will have a form like::
@@ -686,13 +745,13 @@ raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
  *		unsigned seq, idx;
  *
  *		do {
- *			seq = raw_read_seqcount_latch(&latch->seq);
+ *			seq = read_seqcount_latch(&latch->seq);
  *
  *			idx = seq & 0x01;
  *			entry = data_query(latch->data[idx], ...);
  *
  *		// This includes needed smp_rmb()
- *		} while (raw_read_seqcount_latch_retry(&latch->seq, seq));
+ *		} while (read_seqcount_latch_retry(&latch->seq, seq));
  *
  *		return entry;
  *	}
@@ -716,11 +775,31 @@ raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
  *	When data is a dynamic data structure; one should use regular RCU
  *	patterns to manage the lifetimes of the objects within.
  */
-static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
+static __always_inline void write_seqcount_latch_begin(seqcount_latch_t *s)
 {
-	smp_wmb();	/* prior stores before incrementing "sequence" */
-	s->seqcount.sequence++;
-	smp_wmb();      /* increment "sequence" before following stores */
+	kcsan_nestable_atomic_begin();
+	raw_write_seqcount_latch(s);
+}
+
+/**
+ * write_seqcount_latch() - redirect latch readers to even copy
+ * @s: Pointer to seqcount_latch_t
+ */
+static __always_inline void write_seqcount_latch(seqcount_latch_t *s)
+{
+	raw_write_seqcount_latch(s);
+}
+
+/**
+ * write_seqcount_latch_end() - end a seqcount_latch_t write section
+ * @s:		Pointer to seqcount_latch_t
+ *
+ * Marks the end of a seqcount_latch_t writer section, after all copies of the
+ * latch-protected data have been updated.
+ */
+static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
+{
+	kcsan_nestable_atomic_end();
 }
 
 #define __SEQLOCK_UNLOCKED(lockname)					\
@@ -754,11 +833,7 @@ static inline void raw_write_seqcount_latch(seqcount_latch_t *s)
  */
 static inline unsigned read_seqbegin(const seqlock_t *sl)
 {
-	unsigned ret = read_seqcount_begin(&sl->seqcount);
-
-	kcsan_atomic_next(0);  /* non-raw usage, assume closing read_seqretry() */
-	kcsan_flat_atomic_begin();
-	return ret;
+	return read_seqcount_begin(&sl->seqcount);
 }
 
 /**
@@ -774,12 +849,6 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
  */
 static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
 {
-	/*
-	 * Assume not nested: read_seqretry() may be called multiple times when
-	 * completing read critical section.
-	 */
-	kcsan_flat_atomic_end();
-
 	return read_seqcount_retry(&sl->seqcount, start);
 }