From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1423103AbdEYAPL (ORCPT ); Wed, 24 May 2017 20:15:11 -0400 Received: from mx2.suse.de ([195.135.220.15]:56630 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1162174AbdEYAO4 (ORCPT ); Wed, 24 May 2017 20:14:56 -0400 Date: Thu, 25 May 2017 02:14:52 +0200 From: "Luis R. Rodriguez" To: Dmitry Torokhov Cc: "Luis R. Rodriguez" , shuah@kernel.org, jeyu@redhat.com, rusty@rustcorp.com.au, ebiederm@xmission.com, acme@redhat.com, corbet@lwn.net, martin.wilck@suse.com, mmarek@suse.com, pmladek@suse.com, hare@suse.com, rwright@hpe.com, jeffm@suse.com, DSterba@suse.com, fdmanana@suse.com, neilb@suse.com, linux@roeck-us.net, rgoldwyn@suse.com, subashab@codeaurora.org, xypron.glpk@gmx.de, keescook@chromium.org, atomlin@redhat.com, mbenes@suse.cz, paulmck@linux.vnet.ibm.com, dan.j.williams@intel.com, jpoimboe@redhat.com, davem@davemloft.net, mingo@redhat.com, akpm@linux-foundation.org, torvalds@linux-foundation.org, gregkh@linuxfoundation.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 5/6] kmod: preempt on kmod_umh_threads_get() Message-ID: <20170525001452.GS8951@wotan.suse.de> References: <20170519032444.18416-1-mcgrof@kernel.org> <20170519032444.18416-6-mcgrof@kernel.org> <20170519222712.GI19281@dtor-ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170519222712.GI19281@dtor-ws> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 19, 2017 at 03:27:12PM -0700, Dmitry Torokhov wrote: > On Thu, May 18, 2017 at 08:24:43PM -0700, Luis R. Rodriguez wrote: > > In theory it is possible multiple concurrent threads will try to > > kmod_umh_threads_get() and as such atomic_inc(&kmod_concurrent) at > > the same time, therefore enabling a small time during which we've > > bumped kmod_concurrent but have not really enabled work. By using > > preemption we mitigate this a bit. > > > > Preemption is not needed when we kmod_umh_threads_put(). > > > > Signed-off-by: Luis R. Rodriguez > > --- > > kernel/kmod.c | 24 ++++++++++++++++++++++-- > > 1 file changed, 22 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/kmod.c b/kernel/kmod.c > > index 563600fc9bb1..7ea11dbc7564 100644 > > --- a/kernel/kmod.c > > +++ b/kernel/kmod.c > > @@ -113,15 +113,35 @@ static int call_modprobe(char *module_name, int wait) > > > > static int kmod_umh_threads_get(void) > > { > > + int ret = 0; > > + > > + /* > > + * Disabling preemption makes sure that we are not rescheduled here > > + * > > + * Also preemption helps kmod_concurrent is not increased by mistake > > + * for too long given in theory two concurrent threads could race on > > + * atomic_inc() before we atomic_read() -- we know that's possible > > + * and but we don't care, this is not used for object accounting and > > + * is just a subjective threshold. The alternative is a lock. > > + */ > > + preempt_disable(); > > atomic_inc(&kmod_concurrent); > > if (atomic_read(&kmod_concurrent) <= max_modprobes) > > That is very "fancy" way to basically say: > > if (atomic_inc_return(&kmod_concurrent) <= max_modprobes) Do you mean to combine the atomic_inc() and atomic_read() in one as you noted (as that is not a change in this patch), *or* that using a memory barrier here with atomic_inc_return() should suffice to address the same and avoid an explicit preemption enable / disable ? Luis