Subject: Re: [PATCH 2/5] rhashtable: don't hold lock on first table throughout insertion.

From: "Paul E. McKenney" <[email-masked]>
Date: Mon, 30 Jul 2018 21:14:25 -0700

On Tue, Jul 31, 2018 at 10:45:45AM +1000, NeilBrown wrote:
> On Fri, Jul 27 2018, Paul E. McKenney wrote:
> 
> > On Thu, Jul 26, 2018 at 08:18:15PM -0700, Paul E. McKenney wrote:
> >> On Fri, Jul 27, 2018 at 11:04:37AM +1000, NeilBrown wrote:
> >> > On Wed, Jul 25 2018, Paul E. McKenney wrote:
> >> > >> 
> >> > >> Looks good ... except ... naming is hard.
> >> > >> 
> >> > >>  is_after_call_rcu_init()  asserts where in the lifecycle we are,
> >> > >>  is_after_call_rcu() tests where in the lifecycle we are.
> >> > >> 
> >> > >>  The names are similar but the purpose is quite different.
> >> > >>  Maybe s/is_after_call_rcu_init/call_rcu_init/ ??
> >> > >
> >> > > How about rcu_head_init() and rcu_head_after_call_rcu()?
> >> 
> >> Very well, I will pull this change in on my next rebase.
> >
> > Like this?
> 
> Hard to say - unwinding white-space damage in my head is too challenging
> when newlines have been deleted :-(

What???  Don't you like block-structured code?

All kidding aside, how about the following more conventionally formatted
version?

							Thanx, Paul

------------------------------------------------------------------------

commit e3408141ed7d702995b2fdc94703af88aadd226b
Author: Paul E. McKenney <[email-masked]>
Date:   Tue Jul 24 15:28:09 2018 -0700

    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 <[email-masked]>
    Signed-off-by: Paul E. McKenney <[email-masked]>
    [ paulmck: Apply neilb naming feedback. ]

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index e4f821165d0b..4db8bcacc51a 100644
--- 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 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;
 	}