[tip: sched/core] seqlock/latch: Provide raw_read_seqcount_latch_retry()

["tip-bot2 for Peter Zijlstra" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     d16317de9b412aa7bd3598c607112298e36b4352
Gitweb:        https://git.kernel.org/tip/d16317de9b412aa7bd3598c607112298e36b4352
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Fri, 19 May 2023 12:20:59 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 05 Jun 2023 21:11:03 +02:00

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 |  2 +-
 include/linux/seqlock.h      | 15 ++++++++-------
 kernel/printk/printk.c       |  2 +-
 kernel/time/sched_clock.c    |  2 +-
 kernel/time/timekeeping.c    |  4 ++--
 5 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
index 3d1a9e7..6a0999c 100644
--- a/include/linux/rbtree_latch.h
+++ b/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;
 }
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 3926e90..987a59d 100644
--- a/include/linux/seqlock.h
+++ b/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
-read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
+static __always_inline int
+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;
  *	}
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 6a333ad..357a4d1 100644
--- a/kernel/printk/printk.c
+++ b/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;
 }
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 8464c5a..e8f2fb0 100644
--- a/kernel/time/sched_clock.c
+++ b/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)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 09d5949..266d028 100644
--- a/kernel/time/timekeeping.c
+++ b/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;