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. ]

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index e4f821165d0be38b4be42263895c187738468613..4db8bcacc51ae3d84e77157ebe7ad29364e55c96 100644 (file)
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -857,6 +857,46 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
 
 
+/* Has the specified rcu_head structure been handed to call_rcu()? */
+
+/*
+ * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
+ * @rhp: The rcu_head structure to initialize.
+ *
+ * If you intend to invoke rcu_head_after_call_rcu() to test whether a
+ * given rcu_head structure has already been passed to call_rcu(), then
+ * you must also invoke this rcu_head_init() function on it just after
+ * allocating that structure.  Calls to this function must not race with
+ * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
+ */
+static inline void rcu_head_init(struct rcu_head *rhp)
+{
+       rhp->func = (rcu_callback_t)~0L;
+}
+
+/*
+ * rcu_head_after_call_rcu - Has this rcu_head been passed to call_rcu()?
+ * @rhp: The rcu_head structure to test.
+ * @func: The function passed to call_rcu() along with @rhp.
+ *
+ * Returns @true if the @rhp has been passed to call_rcu() with @func,
+ * and @false otherwise.  Emits a warning in any other case, including
+ * the case where @rhp has already been invoked after a grace period.
+ * Calls to this function must not race with callback invocation.  One way
+ * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
+ * in an RCU read-side critical section that includes a read-side fetch
+ * of the pointer to the structure containing @rhp.
+ */
+static inline bool
+rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
+{
+       if (READ_ONCE(rhp->func) == f)
+               return true;
+       WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L);
+       return false;
+}
+
+
 /* Transitional pre-consolidation compatibility definitions. */
 
 static inline void synchronize_rcu_bh(void)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 5dec94509a7e6eaf4327354d380c06d802ba6cc4..4c56c1d98fb3672231458336101e754e155d117b 100644 (file)
--- 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;
        }