Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757221AbaFSDA5 (ORCPT ); Wed, 18 Jun 2014 23:00:57 -0400 Received: from cn.fujitsu.com ([59.151.112.132]:9498 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1757140AbaFSDA4 (ORCPT ); Wed, 18 Jun 2014 23:00:56 -0400 X-IronPort-AV: E=Sophos;i="5.00,735,1396972800"; d="scan'208";a="32125957" Message-ID: <53A25286.1030003@cn.fujitsu.com> Date: Thu, 19 Jun 2014 11:01:26 +0800 From: Lai Jiangshan User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc14 Thunderbird/3.1.4 MIME-Version: 1.0 To: Tejun Heo CC: , , , "Paul E. McKenney" Subject: Re: [PATCH v2 6/6] percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is_zero() References: <1403053685-28240-1-git-send-email-tj@kernel.org> <1403053685-28240-7-git-send-email-tj@kernel.org> <20140619022055.GD20100@mtj.dyndns.org> In-Reply-To: <20140619022055.GD20100@mtj.dyndns.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.167.226.103] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/19/2014 10:20 AM, Tejun Heo wrote: > Now that explicit invocation of percpu_ref_exit() is necessary to free > the percpu counter, we can implement percpu_ref_reinit() which > reinitializes a released percpu_ref. This can be used implement > scalable gating switch which can be drained and then re-opened without > worrying about memory allocation failures. > > percpu_ref_is_zero() is added to be used in a sanity check in > percpu_ref_exit(). As this function will be useful for other purposes > too, make it a public interface. > > v2: Use smp_read_barrier_depends() instead of smp_load_acquire(). We > only need data dep barrier and smp_load_acquire() is stronger and > heavier on some archs. Spotted by Lai Jiangshan. > > Signed-off-by: Tejun Heo > Cc: Kent Overstreet > Cc: Christoph Lameter > Cc: Lai Jiangshan > --- > include/linux/percpu-refcount.h | 19 +++++++++++++++++++ > lib/percpu-refcount.c | 35 +++++++++++++++++++++++++++++++++++ > 2 files changed, 54 insertions(+) > > --- a/include/linux/percpu-refcount.h > +++ b/include/linux/percpu-refcount.h > @@ -67,6 +67,7 @@ struct percpu_ref { > > int __must_check percpu_ref_init(struct percpu_ref *ref, > percpu_ref_func_t *release); > +void percpu_ref_reinit(struct percpu_ref *ref); > void percpu_ref_exit(struct percpu_ref *ref); > void percpu_ref_kill_and_confirm(struct percpu_ref *ref, > percpu_ref_func_t *confirm_kill); > @@ -99,6 +100,9 @@ static inline bool __pcpu_ref_alive(stru > { > unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr); > > + /* paired with smp_store_release() in percpu_ref_reinit() */ > + smp_read_barrier_depends(); > + > if (unlikely(pcpu_ptr & PCPU_REF_DEAD)) > return false; > > @@ -206,4 +210,19 @@ static inline void percpu_ref_put(struct > rcu_read_unlock_sched(); > } > > +/** > + * percpu_ref_is_zero - test whether a percpu refcount reached zero > + * @ref: percpu_ref to test > + * > + * Returns %true if @ref reached zero. > + */ > +static inline bool percpu_ref_is_zero(struct percpu_ref *ref) > +{ > + unsigned __percpu *pcpu_count; > + > + if (__pcpu_ref_alive(ref, &pcpu_count)) > + return false; > + return !atomic_read(&ref->count); > +} > + > #endif > --- a/lib/percpu-refcount.c > +++ b/lib/percpu-refcount.c > @@ -61,6 +61,41 @@ int percpu_ref_init(struct percpu_ref *r > EXPORT_SYMBOL_GPL(percpu_ref_init); > > /** > + * percpu_ref_reinit - re-initialize a percpu refcount > + * @ref: perpcu_ref to re-initialize > + * > + * Re-initialize @ref so that it's in the same state as when it finished > + * percpu_ref_init(). @ref must have been initialized successfully, killed > + * and reached 0 but not exited. > + * > + * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while > + * this function is in progress. > + */ > +void percpu_ref_reinit(struct percpu_ref *ref) > +{ > + unsigned __percpu *pcpu_count = pcpu_count_ptr(ref); > + int cpu; > + > + BUG_ON(!pcpu_count); > + WARN_ON(!percpu_ref_is_zero(ref)); > + > + atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS); > + > + /* > + * Restore per-cpu operation. smp_store_release() is paired with > + * smp_load_acquire() in __pcpu_ref_alive() and guarantees that the s/smp_load_acquire()/smp_read_barrier_depends()/ s/smp_store_release()/smp_mb()/ if you accept my next comment. > + * zeroing is visible to all percpu accesses which can see the > + * following PCPU_REF_DEAD clearing. > + */ > + for_each_possible_cpu(cpu) > + *per_cpu_ptr(pcpu_count, cpu) = 0; > + > + smp_store_release(&ref->pcpu_count_ptr, > + ref->pcpu_count_ptr & ~PCPU_REF_DEAD); I think it would be better if smp_mb() is used. it is documented that smp_read_barrier_depends() and smp_mb() are paired. Not smp_read_barrier_depends() and smp_store_release(). > +} > +EXPORT_SYMBOL_GPL(percpu_ref_reinit); > + > +/** > * percpu_ref_exit - undo percpu_ref_init() > * @ref: percpu_ref to exit > * > -- > 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/ > . > -- 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/