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

[ Upstream commit 3c6b0c1c28184038d90dffe8eb542bedcb8ccf98 ]

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
	srcu_read_lock();
	synchronize_srcu();
	srcu_read_unlock();

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")
Link: https://lore.kernel.org/r/20230927160231.XRCDDSK4@linutronix.de
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 5e5f920..44aab5c 100644 (file)
--- 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 127ef3b..236610e 100644 (file)
--- 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;
 }