Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932108Ab3FLUqf (ORCPT ); Wed, 12 Jun 2013 16:46:35 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48436 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756585Ab3FLUqd (ORCPT ); Wed, 12 Jun 2013 16:46:33 -0400 Date: Wed, 12 Jun 2013 13:46:27 -0700 From: Tejun Heo To: Kent Overstreet Cc: linux-kernel@vger.kernel.org, Rusty Russell , Oleg Nesterov , Christoph Lameter Subject: [PATCH 2/2] percpu-refcount: implement percpu_tryget() along with percpu_ref_kill_and_confirm() Message-ID: <20130612204627.GC15092@htj.dyndns.org> References: <20130612204532.GB15092@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130612204532.GB15092@htj.dyndns.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5578 Lines: 159 >From de3c0749e2c1960afcc433fc5da136b85c8bd896 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Wed, 12 Jun 2013 13:37:42 -0700 Implement percpu_tryget() which succeeds iff the refcount hasn't been killed yet. Because the refcnt is per-cpu, different CPUs may have different perceptions on when the counter has been killed and tryget() may continue to succeed for a while after percpu_ref_kill() returns. For use cases where it's necessary to know when all CPUs start to see the refcnt as dead, percpu_ref_kill_and_confirm() is added. The new function takes an extra argument @confirm_kill which is invoked when the refcnt is guaranteed to be viewed as killed on all CPUs. While this isn't the prettiest interface, it doesn't force synchronous wait and is much safer than requiring the caller to do its own call_rcu(). Signed-off-by: Tejun Heo --- I'll soon post a cgroup patchset which makes use of both functions. Thanks. include/linux/percpu-refcount.h | 50 ++++++++++++++++++++++++++++++++++++++++- lib/percpu-refcount.c | 23 ++++++++++++++----- 2 files changed, 66 insertions(+), 7 deletions(-) diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h index b61bd6f..b2dfa0f 100644 --- a/include/linux/percpu-refcount.h +++ b/include/linux/percpu-refcount.h @@ -63,11 +63,28 @@ struct percpu_ref { */ unsigned __percpu *pcpu_count; percpu_ref_func_t *release; + percpu_ref_func_t *confirm_kill; struct rcu_head rcu; }; int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release); -void percpu_ref_kill(struct percpu_ref *ref); +void percpu_ref_kill_and_confirm(struct percpu_ref *ref, + percpu_ref_func_t *confirm_kill); + +/** + * percpu_ref_kill - drop the initial ref + * @ref: percpu_ref to kill + * + * Must be used to drop the initial ref on a percpu refcount; must be called + * precisely once before shutdown. + * + * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the + * percpu counters and dropping the initial ref. + */ +static inline void percpu_ref_kill(struct percpu_ref *ref) +{ + return percpu_ref_kill_and_confirm(ref, NULL); +} #define PCPU_STATUS_BITS 2 #define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1) @@ -99,6 +116,37 @@ static inline void percpu_ref_get(struct percpu_ref *ref) } /** + * percpu_ref_tryget - try to increment a percpu refcount + * @ref: percpu_ref to try-get + * + * Increment a percpu refcount unless it has already been killed. Returns + * %true on success; %false on failure. + * + * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget + * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be + * used. After the confirm_kill callback is invoked, it's guaranteed that + * no new reference will be given out by percpu_ref_tryget(). + */ +static inline bool percpu_ref_tryget(struct percpu_ref *ref) +{ + unsigned __percpu *pcpu_count; + int ret = false; + + rcu_read_lock(); + + pcpu_count = ACCESS_ONCE(ref->pcpu_count); + + if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) { + __this_cpu_inc(*pcpu_count); + ret = true; + } + + rcu_read_unlock(); + + return ret; +} + +/** * percpu_ref_put - decrement a percpu refcount * @ref: percpu_ref to put * diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c index 9a78e55..4d9519a 100644 --- a/lib/percpu-refcount.c +++ b/lib/percpu-refcount.c @@ -89,6 +89,10 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu) atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count); + /* @ref is viewed as dead on all CPUs, send out kill confirmation */ + if (ref->confirm_kill) + ref->confirm_kill(ref); + /* * Now we're in single atomic_t mode with a consistent refcount, so it's * safe to drop our initial ref: @@ -97,22 +101,29 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu) } /** - * percpu_ref_kill - safely drop initial ref + * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation * @ref: percpu_ref to kill + * @confirm_kill: optional confirmation callback * - * Must be used to drop the initial ref on a percpu refcount; must be called - * precisely once before shutdown. + * Equivalent to percpu_ref_kill() but also schedules kill confirmation if + * @confirm_kill is not NULL. @confirm_kill, which may not block, will be + * called after @ref is seen as dead from all CPUs - all further + * invocations of percpu_ref_tryget() will fail. See percpu_ref_tryget() + * for more details. * - * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the - * percpu counters and dropping the initial ref. + * It's guaranteed that there will be at least one full RCU grace period + * between the invocation of this function and @confirm_kill and the caller + * can piggy-back their RCU release on the callback. */ -void percpu_ref_kill(struct percpu_ref *ref) +void percpu_ref_kill_and_confirm(struct percpu_ref *ref, + percpu_ref_func_t *confirm_kill) { WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD, "percpu_ref_kill() called more than once!\n"); ref->pcpu_count = (unsigned __percpu *) (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD); + ref->confirm_kill = confirm_kill; call_rcu(&ref->rcu, percpu_ref_kill_rcu); } -- 1.8.2.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/