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From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: 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()
Date: Wed, 24 May 2017 17:45:37 -0700
Message-ID: <20170525004537.GA14955@dtor-ws> (raw)
In-Reply-To: <20170525001452.GS8951@wotan.suse.de>

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 <mcgrof@kernel.org>
> > > ---
> > >  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.

Thanks.

-- 
Dmitry

  reply index

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-05-19  3:24 [PATCH 0/6] kmod: few simple enhancements Luis R. Rodriguez
2017-05-19  3:24 ` [PATCH 1/6] kmod: add dynamic max concurrent thread count Luis R. Rodriguez
2017-05-19 20:44   ` Dmitry Torokhov
     [not found]     ` <CAB=NE6XGL24O+JfTNUG0HO4obhDc-v+HyL0SCrQELiZrj2-qNw@mail.gmail.com>
     [not found]       ` <CAB=NE6Wa4Nemh80yaCCwbjrNRLPD+GJMncg12APg9Vq63AWVng@mail.gmail.com>
     [not found]         ` <CAB=NE6Vc6RDAytn2Pkv2V58HFo8ncR0eOHZ3===kbZ2NF78ubg@mail.gmail.com>
     [not found]           ` <CAB=NE6Vqmx=y6muenpuQKynTP=pGWMF8tzoCA0BXD6d63q9wPg@mail.gmail.com>
2017-05-19 21:58             ` Dmitry Torokhov
2017-05-25 16:22               ` Luis R. Rodriguez
2017-05-25 16:38                 ` Dmitry Torokhov
2017-05-25 16:50                   ` Luis R. Rodriguez
2017-05-25 17:30                     ` Dmitry Torokhov
2017-05-25 17:38                       ` Luis R. Rodriguez
2017-05-25 18:06                         ` Luis R. Rodriguez
2017-05-25 18:26                           ` Dmitry Torokhov
2017-05-25 19:01                             ` Luis R. Rodriguez
2017-05-25 21:38                               ` Luis R. Rodriguez
2017-05-19  3:24 ` [PATCH 2/6] module: use list_for_each_entry_rcu() on find_module_all() Luis R. Rodriguez
2017-05-23 14:46   ` Miroslav Benes
2017-05-19  3:24 ` [PATCH 3/6] kmod: provide wrappers for kmod_concurrent inc/dec Luis R. Rodriguez
2017-05-19  3:24 ` [PATCH 4/6] kmod: return -EBUSY if modprobe limit is reached Luis R. Rodriguez
2017-05-19  3:24 ` [PATCH 5/6] kmod: preempt on kmod_umh_threads_get() Luis R. Rodriguez
2017-05-19 22:27   ` Dmitry Torokhov
2017-05-25  0:14     ` Luis R. Rodriguez
2017-05-25  0:45       ` Dmitry Torokhov [this message]
2017-05-25  1:00         ` Luis R. Rodriguez
2017-05-25  2:27           ` Dmitry Torokhov
2017-05-25 11:19             ` Petr Mladek
2017-05-25 15:38               ` Luis R. Rodriguez
2017-05-25 16:42               ` Dmitry Torokhov
2017-05-25 15:18             ` Jessica Yu
2017-05-19  3:24 ` [PATCH 6/6] kmod: use simplified rate limit printk Luis R. Rodriguez
2017-05-19 22:23   ` Dmitry Torokhov
2017-05-23  9:00     ` Petr Mladek
2017-05-26  0:16 ` [PATCH v2 0/5] kmod: help make deterministic Luis R. Rodriguez
2017-05-26  0:16   ` [PATCH v2 1/5] module: use list_for_each_entry_rcu() on find_module_all() Luis R. Rodriguez
2017-05-26  0:16   ` [PATCH v2 2/5] kmod: reduce atomic operations on kmod_concurrent Luis R. Rodriguez
2017-05-26  1:11     ` Dmitry Torokhov
2017-05-26 20:03       ` Luis R. Rodriguez
2017-05-26  0:16   ` [PATCH v2 3/5] kmod: add test driver to stress test the module loader Luis R. Rodriguez
2017-05-26  0:16   ` [PATCH v2 4/5] kmod: add helpers for getting kmod limit Luis R. Rodriguez
2017-05-26  0:56     ` Dmitry Torokhov
2017-05-26 20:27       ` Luis R. Rodriguez
2017-05-26  0:16   ` [PATCH v2 5/5] kmod: throttle kmod thread limit Luis R. Rodriguez
2017-05-26 21:12   ` [PATCH v3 0/4] kmod: help make deterministic Luis R. Rodriguez
2017-05-26 21:12     ` [PATCH v3 1/4] module: use list_for_each_entry_rcu() on find_module_all() Luis R. Rodriguez
2017-05-26 21:12     ` [PATCH v3 2/4] kmod: reduce atomic operations on kmod_concurrent and simplify Luis R. Rodriguez
2017-06-23 19:19       ` [PATCH v4 " Luis R. Rodriguez
2017-06-26 11:36       ` [PATCH v3 " Petr Mladek
2017-05-26 21:12     ` [PATCH v3 3/4] kmod: add test driver to stress test the module loader Luis R. Rodriguez
2017-05-26 21:12     ` [PATCH v3 4/4] kmod: throttle kmod thread limit Luis R. Rodriguez
2017-06-22 15:19       ` Petr Mladek
2017-06-23 16:16         ` Luis R. Rodriguez
2017-06-23 17:56           ` Luis R. Rodriguez
2017-06-23 19:16             ` Luis R. Rodriguez
2017-06-26 10:03               ` Petr Mladek
2017-06-26  9:55           ` Petr Mladek
2017-06-23 19:20       ` [PATCH v4 " Luis R. Rodriguez
2017-06-26 11:38         ` Petr Mladek
2017-06-28 22:11           ` Luis R. Rodriguez
2017-06-20 20:56     ` [PATCH v3 0/4] kmod: help make deterministic Luis R. Rodriguez
2017-06-21  0:23       ` Kees Cook
2017-06-26 21:37         ` Jessica Yu
2017-06-26 22:44           ` Luis R. Rodriguez
2017-06-27  0:27             ` Luis R. Rodriguez
2017-06-27  8:13               ` Petr Mladek
2017-06-27 10:04                 ` Jessica Yu
2017-06-27 15:26             ` Jessica Yu
2017-06-28  0:49               ` Luis R. Rodriguez
2017-06-28 22:31     ` [PATCH v4 0/3] " Luis R. Rodriguez
2017-06-28 22:31       ` [PATCH v4 1/3] MAINTAINERS: give kmod some maintainer love Luis R. Rodriguez
2017-06-28 22:31       ` [PATCH v4 2/3] kmod: add test driver to stress test the module loader Luis R. Rodriguez
2017-06-28 22:31       ` [PATCH v4 3/3] kmod: throttle kmod thread limit Luis R. Rodriguez

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