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 E70F1C10F14 for ; Sun, 21 Apr 2019 21:08:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A52D620857 for ; Sun, 21 Apr 2019 21:08:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726405AbfDUVIB (ORCPT ); Sun, 21 Apr 2019 17:08:01 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35748 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725872AbfDUVIB (ORCPT ); Sun, 21 Apr 2019 17:08:01 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 45B6E81F0F; Sun, 21 Apr 2019 21:08:00 +0000 (UTC) Received: from llong.remote.csb (ovpn-120-184.rdu2.redhat.com [10.10.120.184]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9455160BF4; Sun, 21 Apr 2019 21:07:56 +0000 (UTC) Reply-To: llong@redhat.com Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative To: Waiman Long , Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying References: <20190413172259.2740-1-longman@redhat.com> <20190413172259.2740-15-longman@redhat.com> <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> From: Waiman Long Organization: Red Hat Message-ID: <7b1bfc26-6e90-bd65-ab46-08413acd80e9@redhat.com> Date: Sun, 21 Apr 2019 17:07:56 -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: <57620139-92a3-4a21-56bd-5d6fff23214f@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Sun, 21 Apr 2019 21:08:00 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/19/19 3:39 PM, Waiman Long wrote: > On 04/19/2019 09:15 AM, Peter Zijlstra wrote: >> On Fri, Apr 19, 2019 at 03:03:04PM +0200, Peter Zijlstra wrote: >>> On Fri, Apr 19, 2019 at 02:02:07PM +0200, Peter Zijlstra wrote: >>>> On Fri, Apr 19, 2019 at 12:26:47PM +0200, Peter Zijlstra wrote: >>>>> I thought of a horrible horrible alternative: >>>> Hurm, that's broken as heck. Let me try again. >>> So I can't make that scheme work, it all ends up wanting to have >>> cmpxchg(). >>> >>> Do we have a performance comparison somewhere of xadd vs cmpxchg >>> readers? I tried looking in the old threads, but I can't seem to locate >>> it. >>> >>> We need new instructions :/ Or more clever than I can muster just now. >> In particular, an (unsigned) saturation arithmetic variant of XADD would >> be very nice to have at this point. > I just want to clear about my current scheme. There will be 16 bits > allocated for reader count. I use the MS bit for signaling that there > are too many readers. So the fast path will fail and the readers will be > put into the wait list. This effectively limit readers to 32k-1, but it > doesn't mean the actual reader count cannot go over that. As long as the > actual count is less than 64k, everything should still work perfectly. > IOW, even though we have reached the limit of 32k, we need to pile on an > additional 32k readers to really overflow the count and cause problem. 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 + +/* + * 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 */  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); @@ -1371,11 +1411,12 @@ inline void __down_read(struct rw_semaphore *sem)    static inline int __down_read_killable(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)) { -               if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE, tmp))) +               if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE, wait)))                         return -EINTR;                 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);         } else {