[RFC PATCH] srcu: Use try-lock lockdep annotation for NMI-safe access.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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.

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

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---

The splat:
| ================================
| WARNING: inconsistent lock state
| 6.6.0-rc3-rt5+ #85 Not tainted
| --------------------------------
| inconsistent {INITIAL USE} -> {IN-NMI} usage.
| swapper/0/0 [HC1[1]:SC0[0]:HE0:SE1] takes:
| ffffffff828e6c90 (console_srcu){....}-{0:0}, at: console_srcu_read_lock+0x3a/0x50
| {INITIAL USE} state was registered at:
…
|        CPU0
|        ----
|   lock(console_srcu);
|   <Interrupt>
|     lock(console_srcu);
|
|  *** DEADLOCK ***
|

My guess is that trylock annotation should not apply to
rcu_lock_acquire(). This would distinguish it from from non-NMI safe
srcu_read_lock_nmisafe() and NMI check in rcu_read_unlock() is only
there to survive if accidentally used in-NMI.

 include/linux/rcupdate.h | 6 ++++++
 include/linux/srcu.h     | 2 +-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 5e5f920ade909..44aab5c0bd2c1 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -303,6 +303,11 @@ static inline void rcu_lock_acquire(struct lockdep_map *map)
 	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_);
@@ -317,6 +322,7 @@ int rcu_read_lock_any_held(void);
 #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 127ef3b2e6073..236610e4a8fa5 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -229,7 +229,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp
 
 	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;
 }
 
-- 
2.40.1