From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 2/5] rhashtable: don't hold lock on first table throughout insertion. Date: Tue, 31 Jul 2018 07:44:29 -0700 Message-ID: <20180731144429.GM24813@linux.vnet.ibm.com> References: <87r2jtpqm4.fsf@notabene.neil.brown.name> <20180724225825.GE12945@linux.vnet.ibm.com> <87in53oqzz.fsf@notabene.neil.brown.name> <20180725152250.GN12945@linux.vnet.ibm.com> <87r2jpmqu2.fsf@notabene.neil.brown.name> <20180727031815.GW24813@linux.vnet.ibm.com> <20180727145731.GA2780@linux.vnet.ibm.com> <87zhy8s05i.fsf@notabene.neil.brown.name> <20180731041425.GI24813@linux.vnet.ibm.com> <87lg9sro5r.fsf@notabene.neil.brown.name> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Herbert Xu , Thomas Graf , netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: NeilBrown Return-path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:50484 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1732402AbeGaQZN (ORCPT ); Tue, 31 Jul 2018 12:25:13 -0400 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w6VE92ud130177 for ; Tue, 31 Jul 2018 10:44:33 -0400 Received: from e11.ny.us.ibm.com (e11.ny.us.ibm.com [129.33.205.201]) by mx0b-001b2d01.pphosted.com with ESMTP id 2kjq5afrq6-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 31 Jul 2018 10:44:33 -0400 Received: from localhost by e11.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 31 Jul 2018 10:44:32 -0400 Content-Disposition: inline In-Reply-To: <87lg9sro5r.fsf@notabene.neil.brown.name> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, Jul 31, 2018 at 03:04:48PM +1000, NeilBrown wrote: > On Mon, Jul 30 2018, Paul E. McKenney wrote: > > > 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? > > Wow - it's like I just got new glasses! > Yes - nice an clear and now flaws to be found. Thanks a lot. Now that flaws are to be found, please feel free to report them. ;-) Thanx, Paul > NeilBrown > > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > commit e3408141ed7d702995b2fdc94703af88aadd226b > > Author: Paul E. McKenney > > 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 > > Signed-off-by: Paul E. McKenney > > [ 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; > > }