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

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 27 Jul 2018 07:57:31 -0700

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?

							Thanx, Paul

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

commit 45dfdcaee39bf6dd1785253c78e4805122ee8d93
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
    feedback on naming. ]

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;
	}