rcu: Provide functions for determining if call_rcu() has been invoked

This commit adds rcu_head_init() and rcu_head_after_call_rcu() functions
to help RCU users detect when another CPU has passed the specified
rcu_head structure and function to call_rcu().  The rcu_head_init()
should be invoked before making the structure visible to RCU readers,
and then the rcu_head_after_call_rcu() may be invoked from within
an RCU read-side critical section on an rcu_head structure that
was obtained during a traversal of the data structure in question.
The rcu_head_after_call_rcu() function will return true if the rcu_head
structure has already been passed (with the specified function) to
call_rcu(), otherwise it will return false.

If rcu_head_init() has not been invoked on the rcu_head structure
or if the rcu_head (AKA callback) has already been invoked, then
rcu_head_after_call_rcu() will do WARN_ON_ONCE().

Reported-by: NeilBrown <neilb@suse.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Apply neilb naming feedback. ]

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 5dec94509a7e..4c56c1d98fb3 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -224,6 +224,7 @@ void kfree(const void *);
  */
 static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
 {
+        rcu_callback_t f;
         unsigned long offset = (unsigned long)head->func;
 
         rcu_lock_acquire(&rcu_callback_map);
@@ -234,7 +235,9 @@ static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
                 return true;
         } else {
                 RCU_TRACE(trace_rcu_invoke_callback(rn, head);)
-                head->func(head);
+                f = head->func;
+                WRITE_ONCE(head->func, (rcu_callback_t)0L);
+                f(head);
                 rcu_lock_release(&rcu_callback_map);
                 return false;
         }