From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754010AbcGUUQe (ORCPT ); Thu, 21 Jul 2016 16:16:34 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:52577 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753848AbcGUUQd (ORCPT ); Thu, 21 Jul 2016 16:16:33 -0400 Date: Thu, 21 Jul 2016 13:16:32 -0700 From: Andrew Morton To: Manfred Spraul Cc: "H. Peter Anvin" , Peter Zijlstra , Davidlohr Bueso , LKML , Thomas Gleixner , Ingo Molnar , 1vier1@web.de, felixh@informatik.uni-bremen.de, Subject: Re: [PATCH] ipc/sem.c: Fix complex_count vs. simple op race Message-Id: <20160721131632.51fa6a14f6f81ce1e3ef08d8@linux-foundation.org> In-Reply-To: <1469123695-5661-1-git-send-email-manfred@colorfullife.com> References: <1469123695-5661-1-git-send-email-manfred@colorfullife.com> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 21 Jul 2016 19:54:55 +0200 Manfred Spraul wrote: > Next update: > - switch to smp_store_mb() instead of WRITE_ONCE();smp_mb(); > - introduce SEM_GLOBAL_LOCK instead of magic -1. > - do not use READ_ONCE() for the unlocked&unordered test: > READ_ONCE doesn't make sense for unlocked&unordered code. > - document why smp_mb() is required after spin_lock(). I assume "ipc/sem.c: remove duplicated memory barriers" is still relevant? From: Manfred Spraul Subject: ipc/sem.c: remove duplicated memory barriers With 2c610022711 ("locking/qspinlock: Fix spin_unlock_wait() some more"), memory barriers were added into spin_unlock_wait(). Thus another barrier is not required. And as explained in 055ce0fd1b8 ("locking/qspinlock: Add comments"), spin_lock() provides a barrier so that reads within the critical section cannot happen before the write for the lock is visible. i.e. spin_lock provides an acquire barrier after the write of the lock variable, this barrier pairs with the smp_mb() in complexmode_enter(). Link: http://lkml.kernel.org/r/1468386412-3608-3-git-send-email-manfred@colorfullife.com Signed-off-by: Manfred Spraul Cc: "H. Peter Anvin" Cc: Peter Zijlstra Cc: Davidlohr Bueso Cc: Thomas Gleixner Cc: Ingo Molnar Cc: <1vier1@web.de> Cc: Signed-off-by: Andrew Morton --- ipc/sem.c | 16 ---------------- 1 file changed, 16 deletions(-) diff -puN ipc/sem.c~ipc-semc-remove-duplicated-memory-barriers ipc/sem.c --- a/ipc/sem.c~ipc-semc-remove-duplicated-memory-barriers +++ a/ipc/sem.c @@ -290,14 +290,6 @@ static void complexmode_enter(struct sem sem = sma->sem_base + i; spin_unlock_wait(&sem->lock); } - /* - * spin_unlock_wait() is not a memory barriers, it is only a - * control barrier. The code must pair with spin_unlock(&sem->lock), - * thus just the control barrier is insufficient. - * - * smp_rmb() is sufficient, as writes cannot pass the control barrier. - */ - smp_rmb(); } /* @@ -363,14 +355,6 @@ static inline int sem_lock(struct sem_ar */ spin_lock(&sem->lock); - /* - * See 51d7d5205d33 - * ("powerpc: Add smp_mb() to arch_spin_is_locked()"): - * A full barrier is required: the write of sem->lock - * must be visible before the read is executed - */ - smp_mb(); - if (!smp_load_acquire(&sma->complex_mode)) { /* fast path successful! */ return sops->sem_num; _