Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753093AbdGFH3F (ORCPT ); Thu, 6 Jul 2017 03:29:05 -0400 Received: from mail-wr0-f194.google.com ([209.85.128.194]:35223 "EHLO mail-wr0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751057AbdGFH3C (ORCPT ); Thu, 6 Jul 2017 03:29:02 -0400 Date: Thu, 6 Jul 2017 09:28:58 +0200 From: Ingo Molnar To: Niklas Cassel , Peter Zijlstra Cc: stable@vger.kernel.org, torvalds@linux-foundation.org, tglx@linutronix.de, a.p.zijlstra@chello.nl, ktkhai@virtuozzo.com, hpa@zytor.com, linux-kernel@vger.kernel.org, peterz@infradead.org, linux-tip-commits@vger.kernel.org Subject: Re: [tip:locking/urgent] locking/rwsem-spinlock: Fix EINTR branch in __down_write_common() Message-ID: <20170706072858.yo45chbf6fnccmgw@gmail.com> References: <149762063282.19811.9129615532201147826.stgit@localhost.localdomain> <8a41493e-8752-dd11-0c00-f441b0ac27c6@axis.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8a41493e-8752-dd11-0c00-f441b0ac27c6@axis.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3208 Lines: 85 * Niklas Cassel wrote: > On 07/05/2017 04:27 PM, tip-bot for Kirill Tkhai wrote: > > Commit-ID: a0c4acd2c220376b4e9690e75782d0c0afdaab9f > > Gitweb: http://git.kernel.org/tip/a0c4acd2c220376b4e9690e75782d0c0afdaab9f > > Author: Kirill Tkhai > > AuthorDate: Fri, 16 Jun 2017 16:44:34 +0300 > > Committer: Ingo Molnar > > CommitDate: Wed, 5 Jul 2017 12:26:29 +0200 > > > > locking/rwsem-spinlock: Fix EINTR branch in __down_write_common() > > > > If a writer could been woken up, the above branch > > > > if (sem->count == 0) > > break; > > > > would have moved us to taking the sem. So, it's > > not the time to wake a writer now, and only readers > > are allowed now. Thus, 0 must be passed to __rwsem_do_wake(). > > > > Next, __rwsem_do_wake() wakes readers unconditionally. > > But we mustn't do that if the sem is owned by writer > > in the moment. Otherwise, writer and reader own the sem > > the same time, which leads to memory corruption in > > callers. > > > > rwsem-xadd.c does not need that, as: > > > > 1) the similar check is made lockless there, > > 2) in __rwsem_mark_wake::try_reader_grant we test, > > > > that sem is not owned by writer. > > > > Signed-off-by: Kirill Tkhai > > Acked-by: Peter Zijlstra > > Cc: > > Cc: Linus Torvalds > > Cc: Niklas Cassel > > Cc: Peter Zijlstra (Intel) > > Cc: Peter Zijlstra > > Cc: Thomas Gleixner > > Fixes: 17fcbd590d0c "locking/rwsem: Fix down_write_killable() for CONFIG_RWSEM_GENERIC_SPINLOCK=y" > > Link: http://lkml.kernel.org/r/149762063282.19811.9129615532201147826.stgit@localhost.localdomain > > Signed-off-by: Ingo Molnar > > --- > > kernel/locking/rwsem-spinlock.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c > > index c65f798..20819df 100644 > > --- a/kernel/locking/rwsem-spinlock.c > > +++ b/kernel/locking/rwsem-spinlock.c > > @@ -231,8 +231,8 @@ int __sched __down_write_common(struct rw_semaphore *sem, int state) > > > > out_nolock: > > list_del(&waiter.list); > > - if (!list_empty(&sem->wait_list)) > > - __rwsem_do_wake(sem, 1); > > + if (!list_empty(&sem->wait_list) && sem->count >= 0) > > + __rwsem_do_wake(sem, 0); > > raw_spin_unlock_irqrestore(&sem->wait_lock, flags); > > > > return -EINTR; > > > > For the record, there is actually a v2 of this: > > http://marc.info/?l=linux-kernel&m=149866422128912 Hm, so I missed that because it was within the discussion - please post v2 patches with a new subject line next time around. But I also disagree with -v2 mildly: in practice a >= test has the same CPU overhead as a > test, and if we rely on the earlier "sem->count == 0" test then we should also comment on that. It's more straightforward to just do the canonical sem->count >= 0 test that we do elsewhere in the rwsem-spinlock code. PeterZ, what's your preference? Thanks, Ingo