seqlock/latch: Provide raw_read_seqcount_latch_retry()

The read side of seqcount_latch consists of:

  do {
    seq = raw_read_seqcount_latch(&latch->seq);
    ...
  } while (read_seqcount_latch_retry(&latch->seq, seq));

which is asymmetric in the raw_ department, and sure enough,
read_seqcount_latch_retry() includes (explicit) instrumentation where
raw_read_seqcount_latch() does not.

This inconsistency becomes a problem when trying to use it from
noinstr code. As such, fix it by renaming and re-implementing
raw_read_seqcount_latch_retry() without the instrumentation.

Specifically the instrumentation in question is kcsan_atomic_next(0)
in do___read_seqcount_retry(). Loosing this annotation is not a
problem because raw_read_seqcount_latch() does not pass through
kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Michael Kelley <mikelley@microsoft.com>  # Hyper-V
Link: https://lore.kernel.org/r/20230519102715.233598176@infradead.org

include/linux/rbtree_latch.h

@@ -206,7 +206,7 @@ latch_tree_find(void *key, struct latch_tree_root *root,
 	do {
 		seq = raw_read_seqcount_latch(&root->seq);
 		node = __lt_find(key, root, seq & 1, ops->comp);
-	} while (read_seqcount_latch_retry(&root->seq, seq));
+	} while (raw_read_seqcount_latch_retry(&root->seq, seq));
 
 	return node;
 }

include/linux/seqlock.h

@@ -671,9 +671,9 @@ typedef struct {
  *
  * 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().
+ * with raw_read_seqcount_latch_retry().
  */
-static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
+static __always_inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
 {
 	/*
 	 * Pairs with the first smp_wmb() in raw_write_seqcount_latch().
@@ -683,16 +683,17 @@ static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
 }
 
 /**
- * read_seqcount_latch_retry() - end a seqcount_latch_t read section
+ * 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()
  *
  * Return: true if a read section retry is required, else false
  */
-static inline int
+static __always_inline int
-read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
+raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
 {
-	return read_seqcount_retry(&s->seqcount, start);
+	smp_rmb();
+	return unlikely(READ_ONCE(s->seqcount.sequence) != start);
 }
 
 /**
@@ -752,7 +753,7 @@ read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
  *			entry = data_query(latch->data[idx], ...);
  *
  *		// This includes needed smp_rmb()
- *		} while (read_seqcount_latch_retry(&latch->seq, seq));
+ *		} while (raw_read_seqcount_latch_retry(&latch->seq, seq));
  *
  *		return entry;
  *	}

kernel/printk/printk.c

@@ -528,7 +528,7 @@ static u64 latched_seq_read_nolock(struct latched_seq *ls)
 		seq = raw_read_seqcount_latch(&ls->latch);
 		idx = seq & 0x1;
 		val = ls->val[idx];
-	} while (read_seqcount_latch_retry(&ls->latch, seq));
+	} while (raw_read_seqcount_latch_retry(&ls->latch, seq));
 
 	return val;
 }

kernel/time/sched_clock.c

@@ -77,7 +77,7 @@ notrace struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
 
 notrace int sched_clock_read_retry(unsigned int seq)
 {
-	return read_seqcount_latch_retry(&cd.seq, seq);
+	return raw_read_seqcount_latch_retry(&cd.seq, seq);
 }
 
 unsigned long long notrace sched_clock(void)

kernel/time/timekeeping.c

@@ -450,7 +450,7 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
 		tkr = tkf->base + (seq & 0x01);
 		now = ktime_to_ns(tkr->base);
 		now += fast_tk_get_delta_ns(tkr);
-	} while (read_seqcount_latch_retry(&tkf->seq, seq));
+	} while (raw_read_seqcount_latch_retry(&tkf->seq, seq));
 
 	return now;
 }
@@ -566,7 +566,7 @@ static __always_inline u64 __ktime_get_real_fast(struct tk_fast *tkf, u64 *mono)
 		basem = ktime_to_ns(tkr->base);
 		baser = ktime_to_ns(tkr->base_real);
 		delta = fast_tk_get_delta_ns(tkr);
-	} while (read_seqcount_latch_retry(&tkf->seq, seq));
+	} while (raw_read_seqcount_latch_retry(&tkf->seq, seq));
 
 	if (mono)
 		*mono = basem + delta;