[PATCH 17/48] rcu: Update lockdep while in RCU read-side critical section

[Uladzislau Rezki (Sony)]

With Ankur's lazy-/auto-preemption patches applied and with a
lazy-preemptible kernel in combination with a non-preemptible RCU,
lockdep sometimes complains about context switches within RCU read-side
critical sections.  This is a false positive due to rcu_read_unlock()
updating lockdep state too late:

	__release(RCU);
	__rcu_read_unlock();
	// Context switch here results in lockdep false positive!!!
	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */

Although this complaint could also happen with preemptible RCU
in a preemptible kernel, the odds of that happening aer quite low.
In constrast, with non-preemptible RCU, a long critical section has a
high probability of performing a context switch from the preempt_enable()
in __rcu_read_unlock().

The fix is straightforward, just move the rcu_lock_release()
within rcu_read_unlock() to obtain the reverse order from that of
rcu_read_lock():

	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
	__release(RCU);
	__rcu_read_unlock();

This commit makes this change.

Co-developed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Co-developed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 include/linux/rcupdate.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 17d7ed5f3ae6..2c54750e36a0 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -809,9 +809,9 @@ static inline void rcu_read_unlock(void)
 {
 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
 			 "rcu_read_unlock() used illegally while idle");
+	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
 	__release(RCU);
 	__rcu_read_unlock();
-	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
 }
 
 /**
-- 
2.39.2