From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 898A8C10F14 for ; Tue, 23 Apr 2019 14:31:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4EE7B2175B for ; Tue, 23 Apr 2019 14:31:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728106AbfDWObs (ORCPT ); Tue, 23 Apr 2019 10:31:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43254 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726421AbfDWObr (ORCPT ); Tue, 23 Apr 2019 10:31:47 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D708C3087930; Tue, 23 Apr 2019 14:31:46 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-85.bos.redhat.com [10.18.17.85]) by smtp.corp.redhat.com (Postfix) with ESMTP id 702FF60141; Tue, 23 Apr 2019 14:31:43 +0000 (UTC) Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative To: Peter Zijlstra Cc: Waiman Long , Ingo Molnar , Will Deacon , Thomas Gleixner , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying References: <20190418135151.GB12232@hirez.programming.kicks-ass.net> <20190418144036.GE12232@hirez.programming.kicks-ass.net> <4cbd3c18-c9c0-56eb-4e01-ee355a69057a@redhat.com> <20190419102647.GP7905@worktop.programming.kicks-ass.net> <20190419120207.GO4038@hirez.programming.kicks-ass.net> <20190419130304.GV14281@hirez.programming.kicks-ass.net> <20190419131522.GW14281@hirez.programming.kicks-ass.net> <57620139-92a3-4a21-56bd-5d6fff23214f@redhat.com> <7b1bfc26-6e90-bd65-ab46-08413acd80e9@redhat.com> <20190423141714.GO11158@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: Date: Tue, 23 Apr 2019 10:31:42 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190423141714.GO11158@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Tue, 23 Apr 2019 14:31:47 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/23/19 10:17 AM, Peter Zijlstra wrote: > On Sun, Apr 21, 2019 at 05:07:56PM -0400, Waiman Long wrote: > >> How about the following chunks to disable preemption temporarily for the >> increment-check-decrement sequence? >> >> diff --git a/include/linux/preempt.h b/include/linux/preempt.h >> index dd92b1a93919..4cc03ac66e13 100644 >> --- a/include/linux/preempt.h >> +++ b/include/linux/preempt.h >> @@ -250,6 +250,8 @@ do { \ >>  #define preempt_enable_notrace()               barrier() >>  #define preemptible()                          0 >>   >> +#define __preempt_disable_nop  /* preempt_disable() is nop */ >> + >>  #endif /* CONFIG_PREEMPT_COUNT */ >>   >>  #ifdef MODULE >> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c >> index 043fd29b7534..54029e6af17b 100644 >> --- a/kernel/locking/rwsem.c >> +++ b/kernel/locking/rwsem.c >> @@ -256,11 +256,64 @@ static inline struct task_struct >> *rwsem_get_owner(struct r >>         return (struct task_struct *) (cowner >>                 ? cowner | (sowner & RWSEM_NONSPINNABLE) : sowner); >>  } >> + >> +/* >> + * If __preempt_disable_nop is defined, calling preempt_disable() and >> + * preempt_enable() directly is the most efficient way. Otherwise, it may >> + * be more efficient to disable and enable interrupt instead for disabling >> + * preemption tempoarily. >> + */ >> +#ifdef __preempt_disable_nop >> +#define disable_preemption()   preempt_disable() >> +#define enable_preemption()    preempt_enable() >> +#else >> +#define disable_preemption()   local_irq_disable() >> +#define enable_preemption()    local_irq_enable() >> +#endif > I'm not aware of an architecture where disabling interrupts is faster > than disabling preemption. I have actually done some performance test measuring the effects of disabling interrupt and preemption on readers (on x86-64 system).   Threads    Before patch    Disable irq    Disable preemption   -------    ------------    -----------    ------------------      1          9,088          8,766           9,172      2          9,296          9,169           8,707      4         11,192         11,205          10,712      8         11,329         11,332          11,213 For uncontended case, disable interrupt is slower. The slowdown is gone once the rwsem becomes contended. So it may not be a good idea to disable interrupt as a proxy of disabling preemption. BTW, preemption count is not enabled in typical distro production kernels like RHEL. So preempt_disable() is just a barrier. It is turned on in the debug kernel, though. >> +/* >> + * When the owner task structure pointer is merged into couunt, less bits >> + * will be available for readers. Therefore, there is a very slight chance >> + * that the reader count may overflow. We try to prevent that from >> happening >> + * by checking for the MS bit of the count and failing the trylock attempt >> + * if this bit is set. >> + * >> + * With preemption enabled, there is a remote possibility that preemption >> + * can happen in the narrow timing window between incrementing and >> + * decrementing the reader count and the task is put to sleep for a >> + * considerable amount of time. If sufficient number of such unfortunate >> + * sequence of events happen, we may still overflow the reader count. >> + * To avoid such possibility, we have to disable preemption for the >> + * whole increment-check-decrement sequence. >> + * >> + * The function returns true if there are too many readers and the count >> + * has already been properly decremented so the reader must go directly >> + * into the wait list. >> + */ >> +static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cnt) >> +{ >> +       bool wait = false;      /* Wait now flag */ >> + >> +       disable_preemption(); >> +       *cnt = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, >> &sem->count); >> +       if (unlikely(*cnt < 0)) { >> +               atomic_long_add(-RWSEM_READER_BIAS, &sem->count); >> +               wait = true; >> +       } >> +       enable_preemption(); >> +       return wait; >> +} >>  #else /* !CONFIG_RWSEM_OWNER_COUNT */ > This also means you have to ensure CONFIG_NR_CPUS < 32K for > RWSEM_OWNER_COUNT. Yes, that can be done. > >>  static inline struct task_struct *rwsem_get_owner(struct rw_semaphore *sem) >>  { >>         return READ_ONCE(sem->owner); >>  } >> + >> +static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cnt) >> +{ >> +       *cnt = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, >> &sem->count); >> +       return false; >> +} >>  #endif /* CONFIG_RWSEM_OWNER_COUNT */ >>   >>  /* >> @@ -981,32 +1034,18 @@ static inline void clear_wr_nonspinnable(struct >> rw_semaph >>   * Wait for the read lock to be granted >>   */ >>  static struct rw_semaphore __sched * >> -rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count) >> +rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, const >> bool wait) >>  { >> -       long adjustment = -RWSEM_READER_BIAS; >> +       long count, adjustment = -RWSEM_READER_BIAS; >>         bool wake = false; >>         struct rwsem_waiter waiter; >>         DEFINE_WAKE_Q(wake_q); >>   >> -       if (unlikely(count < 0)) { >> +       if (unlikely(wait)) { >>                 /* >> -                * The sign bit has been set meaning that too many active >> -                * readers are present. We need to decrement reader count & >> -                * enter wait queue immediately to avoid overflowing the >> -                * reader count. >> -                * >> -                * As preemption is not disabled, there is a remote >> -                * possibility that preemption can happen in the narrow >> -                * timing window between incrementing and decrementing >> -                * the reader count and the task is put to sleep for a >> -                * considerable amount of time. If sufficient number >> -                * of such unfortunate sequence of events happen, we >> -                * may still overflow the reader count. It is extremely >> -                * unlikey, though. If this is a concern, we should consider >> -                * disable preemption during this timing window to make >> -                * sure that such unfortunate event will not happen. >> +                * The reader count has already been decremented and the >> +                * reader should go directly into the wait list now. >>                  */ >> -               atomic_long_add(-RWSEM_READER_BIAS, &sem->count); >>                 adjustment = 0; >>                 goto queue; >>         } >> @@ -1358,11 +1397,12 @@ static struct rw_semaphore >> *rwsem_downgrade_wake(struct >>   */ >>  inline void __down_read(struct rw_semaphore *sem) >>  { >> -       long tmp = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, >> -                                                &sem->count); >> +       long tmp; >> +       bool wait; >>   >> +       wait = rwsem_read_trylock(sem, &tmp); >>         if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) { >> -               rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, tmp); >> +               rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, wait); >>                 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem); >>         } else { >>                 rwsem_set_reader_owned(sem); > I think I prefer that function returning/taking the bias/adjustment > value instead of a bool, if it is all the same. Sure, I can do that. Cheers, Longman