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: Mon, 30 Jul 2018 21:14:25 -0700 Message-ID: <20180731041425.GI24813@linux.vnet.ibm.com> References: <87h8kqrhi0.fsf@notabene.neil.brown.name> <20180723205625.GZ12945@linux.vnet.ibm.com> <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> 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: Content-Disposition: inline In-Reply-To: <87zhy8s05i.fsf@notabene.neil.brown.name> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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 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; }