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

[Sebastian Andrzej Siewior]

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.

Fixes: f0f44752f5f61 ("rcu: Annotate SRCU's update-side lockdep dependencies")
Link: https://lore.kernel.org/r/20230927160231.XRCDDSK4@xxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---

This is a repost of
	https://lore.kernel.org/r/20230927160231.XRCDDSK4@xxxxxxxxxxxxx

Based on the discussion there I *think* this is preferred over the NMI
check in lock_acquire().
But then PeterZ also pointed out that he has a problem with
	f0f44752f5f61 ("rcu: Annotate SRCU's update-side lockdep dependencies")

because trace_.*_rcuidle machinery. This looks okay because the _rcuidle
part is using SRCU and the rcu_dereference_raw() tracepoint_func is
using RCU + SRCU in its free part.

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

--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -301,6 +301,11 @@ static inline void rcu_lock_acquire(stru
 	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 @@ 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)
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -229,7 +229,7 @@ static inline int srcu_read_lock_nmisafe
 
 	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;
 }