We recently got a report of a crash [1] with misuse of call_rcu().

Instead of crashing the kernel, a warning and graceful return is better.
 [1] https://lore.kernel.org/all/aEnVuzK7VhGSizWj@pc636/
 
 Uladzislau Rezki (Sony) (1):
       rcu: Return early if callback is not specified
 
  kernel/rcu/tree.c | 4 ++++
  1 file changed, 4 insertions(+)
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Merge tag 'rcu/fixes-for-6.16-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/rcu/linux

Pull RCU fix from Joel Fernandes:
 "We recently got a report of a crash [1] with misuse of call_rcu().

  Instead of crashing the kernel, a warning and graceful return is
  better:

   - rcu: Return early if callback is not specified (Uladzislau Rezki)"

Link: https://lore.kernel.org/all/aEnVuzK7VhGSizWj@pc636/ [1]

* tag 'rcu/fixes-for-6.16-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/rcu/linux:
  rcu: Return early if callback is not specified

kernel/rcu/tree.c

@@ -3072,6 +3072,10 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
 	/* Misaligned rcu_head! */
 	WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
 
+	/* Avoid NULL dereference if callback is NULL. */
+	if (WARN_ON_ONCE(!func))
+		return;
+
 	if (debug_rcu_head_queue(head)) {
 		/*
 		 * Probable double call_rcu(), so leak the callback.