From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:53980 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754692AbbIKWu0 (ORCPT ); Fri, 11 Sep 2015 18:50:26 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Manfred Spraul , Oleg Nesterov , "Peter Zijlstra (Intel)" , "Paul E. McKenney" , Kirill Tkhai , Ingo Molnar , Josh Poimboeuf , Davidlohr Bueso , Andrew Morton , Linus Torvalds Subject: [PATCH 4.1 02/78] ipc/sem.c: update/correct memory barriers Date: Fri, 11 Sep 2015 15:49:03 -0700 Message-Id: <20150911224607.031746973@linuxfoundation.org> In-Reply-To: <20150911224606.758437370@linuxfoundation.org> References: <20150911224606.758437370@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Sender: stable-owner@vger.kernel.org List-ID: 4.1-stable review patch. If anyone has any objections, please let me know. ------------------ From: Manfred Spraul commit 3ed1f8a99d70ea1cd1508910eb107d0edcae5009 upstream. sem_lock() did not properly pair memory barriers: !spin_is_locked() and spin_unlock_wait() are both only control barriers. The code needs an acquire barrier, otherwise the cpu might perform read operations before the lock test. As no primitive exists inside and since it seems noone wants another primitive, the code creates a local primitive within ipc/sem.c. With regards to -stable: The change of sem_wait_array() is a bugfix, the change to sem_lock() is a nop (just a preprocessor redefinition to improve the readability). The bugfix is necessary for all kernels that use sem_wait_array() (i.e.: starting from 3.10). Signed-off-by: Manfred Spraul Reported-by: Oleg Nesterov Acked-by: Peter Zijlstra (Intel) Cc: "Paul E. McKenney" Cc: Kirill Tkhai Cc: Ingo Molnar Cc: Josh Poimboeuf Cc: Davidlohr Bueso Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- ipc/sem.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) --- a/ipc/sem.c +++ b/ipc/sem.c @@ -253,6 +253,16 @@ static void sem_rcu_free(struct rcu_head } /* + * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they + * are only control barriers. + * The code must pair with spin_unlock(&sem->lock) or + * spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient. + * + * smp_rmb() is sufficient, as writes cannot pass the control barrier. + */ +#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb() + +/* * Wait until all currently ongoing simple ops have completed. * Caller must own sem_perm.lock. * New simple ops cannot start, because simple ops first check @@ -275,6 +285,7 @@ static void sem_wait_array(struct sem_ar sem = sma->sem_base + i; spin_unlock_wait(&sem->lock); } + ipc_smp_acquire__after_spin_is_unlocked(); } /* @@ -327,13 +338,12 @@ static inline int sem_lock(struct sem_ar /* Then check that the global lock is free */ if (!spin_is_locked(&sma->sem_perm.lock)) { /* - * The ipc object lock check must be visible on all - * cores before rechecking the complex count. Otherwise - * we can race with another thread that does: + * We need a memory barrier with acquire semantics, + * otherwise we can race with another thread that does: * complex_count++; * spin_unlock(sem_perm.lock); */ - smp_rmb(); + ipc_smp_acquire__after_spin_is_unlocked(); /* * Now repeat the test of complex_count: