srcu: Use try-lock lockdep annotation for NMI-safe access.

It is claimed that srcu_read_lock_nmisafe() NMI-safe. However it
triggers a lockdep if used from NMI because lockdep expects a deadlock
since nothing disables NMIs while the lock is acquired.

This is because commit f0f44752f5f61 ("rcu: Annotate SRCU's update-side
lockdep dependencies") annotates synchronize_srcu() as a write lock
usage. This helps to detect a deadlocks such as

The side effect is that the lock srcu_struct now has a USED usage in normal
contexts, so it conflicts with a USED_READ usage in NMI. But this shouldn't
cause a real deadlock because the write lock usage from synchronize_srcu()
is a fake one and only used for read/write deadlock detection.

Use a try-lock annotation for srcu_read_lock_nmisafe() to avoid lockdep
complains if used from NMI.

Fixes: f0f44752f5f6 ("rcu: Annotate SRCU's update-side lockdep dependencies")
Reviewed-by: Boqun Feng <>
Signed-off-by: Sebastian Andrzej Siewior <>
Signed-off-by: Paul E. McKenney <>
Signed-off-by: Neeraj Upadhyay (AMD) <>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index f7206b2..31d523c 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -301,6 +301,11 @@
 	lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
+static inline void rcu_try_lock_acquire(struct lockdep_map *map)
+	lock_acquire(map, 0, 1, 2, 0, NULL, _THIS_IP_);
 static inline void rcu_lock_release(struct lockdep_map *map)
 	lock_release(map, _THIS_IP_);
@@ -315,6 +320,7 @@
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 # define rcu_lock_acquire(a)		do { } while (0)
+# define rcu_try_lock_acquire(a)	do { } while (0)
 # define rcu_lock_release(a)		do { } while (0)
 static inline int rcu_read_lock_held(void)
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 127ef3b..236610e 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -229,7 +229,7 @@
 	srcu_check_nmi_safety(ssp, true);
 	retval = __srcu_read_lock_nmisafe(ssp);
-	rcu_lock_acquire(&ssp->dep_map);
+	rcu_try_lock_acquire(&ssp->dep_map);
 	return retval;