[PATCH RFC rcu] Inform KCSAN of one-byte cmpxchg() in rcu_trc_cmpxchg_need_qs()

["Paul E. McKenney" <paulmck@kernel.org>]

Tasks Trace RCU needs a single-byte cmpxchg(), but no such thing exists.
Therefore, rcu_trc_cmpxchg_need_qs() emulates one using field substitution
and a four-byte cmpxchg(), such that the other three bytes are always
atomically updated to their old values.  This works, but results in
false-positive KCSAN failures because as far as KCSAN knows, this
cmpxchg() operation is updating all four bytes.

This commit therefore encloses the cmpxchg() in a data_race() and adds
a single-byte instrument_atomic_read_write(), thus telling KCSAN exactly
what is going on so as to avoid the false positives.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Marco Elver <elver@google.com>

---

Is this really the right way to do this?

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index d5319bbe8c982..e83adcdb49b5f 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1460,6 +1460,7 @@ static void rcu_st_need_qs(struct task_struct *t, u8 v)
 /*
  * Do a cmpxchg() on ->trc_reader_special.b.need_qs, allowing for
  * the four-byte operand-size restriction of some platforms.
+ *
  * Returns the old value, which is often ignored.
  */
 u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
@@ -1471,7 +1472,13 @@ u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
 	if (trs_old.b.need_qs != old)
 		return trs_old.b.need_qs;
 	trs_new.b.need_qs = new;
-	ret.s = cmpxchg(&t->trc_reader_special.s, trs_old.s, trs_new.s);
+
+	// Although cmpxchg() appears to KCSAN to update all four bytes,
+	// only the .b.need_qs byte actually changes.
+	instrument_atomic_read_write(&t->trc_reader_special.b.need_qs,
+				     sizeof(t->trc_reader_special.b.need_qs));
+	ret.s = data_race(cmpxchg(&t->trc_reader_special.s, trs_old.s, trs_new.s));
+
 	return ret.b.need_qs;
 }
 EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);