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, 24 Jul 2018 15:58:25 -0700 Message-ID: <20180724225825.GE12945@linux.vnet.ibm.com> References: <153086169828.24852.10332573315056854948.stgit@noble> <153086175009.24852.7782466383056542839.stgit@noble> <20180720075409.kfckhodsnvktift7@gondor.apana.org.au> <20180720144152.GW12945@linux.vnet.ibm.com> <87muulqq8q.fsf@notabene.neil.brown.name> <20180722215446.GH12945@linux.vnet.ibm.com> <87h8kqrhi0.fsf@notabene.neil.brown.name> <20180723205625.GZ12945@linux.vnet.ibm.com> <87r2jtpqm4.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 mx0a-001b2d01.pphosted.com ([148.163.156.1]:42912 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388736AbeGYAHO (ORCPT ); Tue, 24 Jul 2018 20:07:14 -0400 Received: from pps.filterd (m0098396.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w6OMsCEL010467 for ; Tue, 24 Jul 2018 18:58:30 -0400 Received: from e15.ny.us.ibm.com (e15.ny.us.ibm.com [129.33.205.205]) by mx0a-001b2d01.pphosted.com with ESMTP id 2ke9knrhgx-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 24 Jul 2018 18:58:30 -0400 Received: from localhost by e15.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 24 Jul 2018 18:58:28 -0400 Content-Disposition: inline In-Reply-To: <87r2jtpqm4.fsf@notabene.neil.brown.name> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, Jul 24, 2018 at 07:52:03AM +1000, NeilBrown wrote: > On Mon, Jul 23 2018, Paul E. McKenney wrote: > > > On Mon, Jul 23, 2018 at 09:13:43AM +1000, NeilBrown wrote: > >> On Sun, Jul 22 2018, Paul E. McKenney wrote: > >> > > >> > One issue is that the ->func pointer can legitimately be NULL while on > >> > RCU's callback lists. This happens when someone invokes kfree_rcu() > >> > with the rcu_head structure at the beginning of the enclosing structure. > >> > I could add an offset to avoid this, or perhaps the kmalloc() folks > >> > could be persuaded Rao Shoaib's patch moving kfree_rcu() handling to > >> > the slab allocators, so that RCU only ever sees function pointers in > >> > the ->func field. > >> > > >> > Either way, this should be hidden behind an API to allow adjustments > >> > to be made if needed. Maybe something like is_after_call_rcu()? > >> > This would (for example) allow debug-object checks to be used to catch > >> > check-after-free bugs. > >> > > >> > Would something of that sort work for you? > >> > >> Yes, if you could provide an is_after_call_rcu() API, that would > >> perfectly suit my use-case. > > > > After beating my head against the object-debug code a bit, I have to ask > > if it would be OK for you if the is_after_call_rcu() API also takes the > > function that was passed to RCU. > > Sure. It feels a bit clumsy, but I can see it could be easier to make > robust. > So yes: I'm fine with pass the same function and rcu_head to both > call_rcu() and is_after_call_rcu(). Actually, when I say it like that, > it seems less clumsy :-) How about like this? (It needs refinements, like lockdep, but should get the gist.) Thanx, Paul ------------------------------------------------------------------------ commit 5aa0ebf4799b8bddbbd0124db1c008526e99fc7c 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 is_after_call_rcu() and is_after_call_rcu_init() functions to help RCU users detect when another CPU has passed the specified rcu_head structure and function to call_rcu(). The is_after_call_rcu_init() should be invoked before making the structure visible to RCU readers, and then the is_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 is_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 is_after_call_rcu_init() has not been invoked on the rcu_head structure or if the rcu_head (AKA callback) has already been invoked, then is_after_call_rcu() will do WARN_ON_ONCE(). Reported-by: NeilBrown Signed-off-by: Paul E. McKenney diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index e4f821165d0b..82e5a91539b5 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -857,6 +857,45 @@ 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()? */ + +/* + * is_after_call_rcu_init - Initialize rcu_head for is_after_call_rcu() + * @rhp: The rcu_head structure to initialize. + * + * If you intend to invoke is_after_call_rcu() to test whether a given + * rcu_head structure has already been passed to call_rcu(), then you must + * also invoke this is_after_call_rcu_init() function on it just after + * allocating that structure. Calls to this function must not race with + * calls to call_rcu(), is_after_call_rcu(), or callback invocation. + */ +static inline void is_after_call_rcu_init(struct rcu_head *rhp) +{ + rhp->func = (rcu_callback_t)~0L; +} + +/* + * is_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 is_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 is_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; }