From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762180AbdEYPiV (ORCPT ); Thu, 25 May 2017 11:38:21 -0400 Received: from mx2.suse.de ([195.135.220.15]:40107 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751871AbdEYPiR (ORCPT ); Thu, 25 May 2017 11:38:17 -0400 Date: Thu, 25 May 2017 17:38:12 +0200 From: "Luis R. Rodriguez" To: Petr Mladek Cc: Dmitry Torokhov , "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, 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: <20170525153812.GU8951@wotan.suse.de> References: <20170519032444.18416-1-mcgrof@kernel.org> <20170519032444.18416-6-mcgrof@kernel.org> <20170519222712.GI19281@dtor-ws> <20170525001452.GS8951@wotan.suse.de> <20170525004537.GA14955@dtor-ws> <20170525010017.GT8951@wotan.suse.de> <20170525022738.GA30589@dtor-ws> <20170525111931.GF26699@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170525111931.GF26699@pathway.suse.cz> 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 Thu, May 25, 2017 at 01:19:31PM +0200, Petr Mladek wrote: > On Wed 2017-05-24 19:27:38, Dmitry Torokhov wrote: > > On Thu, May 25, 2017 at 03:00:17AM +0200, Luis R. Rodriguez wrote: > > > On Wed, May 24, 2017 at 05:45:37PM -0700, Dmitry Torokhov wrote: > > > > On Thu, May 25, 2017 at 02:14:52AM +0200, Luis R. Rodriguez wrote: > > > > > 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 ? > > > > > > > > I am saying that atomic_inc_return() will avoid situation where you have > > > > more than one threads incrementing the counter and believing that they > > > > are [not] allowed to start modprobe. > > > > > > > > I have no idea why you think preempt_disable() would help here. It only > > > > ensures that current thread will not be preempted between the point > > > > where you update the counter and where you check the result. It does not > > > > stop interrupts nor does it affect other threads that might be updating > > > > the same counter. > > > > > > The preemption was inspired by __module_get() and try_module_get(), was that > > > rather silly ? > > > > As far as I can see prrempt_disable() was needed in __module_get() when > > modules user per-cpu refcounts: you did not want to move away from CPU > > while manipulating refcount. > > > > Now that modules use simple atomics for refcounting I think these > > preempt_disable() and preempt_enable() can be removed. > > preempt_disable() still might be useful because you do the > atomic_dec() when you reach the limit. > > By other words, you have three operations that should be atomic: > inc, read, and dec. atomic_inc_return() covers only two of them. > > Hmm, a solution might be to use atomic_dec_if_positive(). > I would kmod_concurrent to something like kmod_concurrent_allowed, > intialize it with the maximum allowed number. Then you could do: > > static int kmod_umh_threads_get(void) > { > if (atomic_dec_if_positive(kmod_concurrent_available) < 0) > return -EBUSY; > return 0; > } I like this much more, thanks! Luis