From: Ming Lei <ming.lei@redhat.com>
To: Miroslav Benes <mbenes@suse.cz>
Cc: Luis Chamberlain <mcgrof@kernel.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
tj@kernel.org, gregkh@linuxfoundation.org,
akpm@linux-foundation.org, minchan@kernel.org, jeyu@kernel.org,
shuah@kernel.org, bvanassche@acm.org, dan.j.williams@intel.com,
joe@perches.com, tglx@linutronix.de, keescook@chromium.org,
rostedt@goodmis.org, linux-spdx@vger.kernel.org,
linux-doc@vger.kernel.org, linux-block@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
linux-kernel@vger.kernel.org, live-patching@vger.kernel.org
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate
Date: Wed, 20 Oct 2021 18:09:51 +0800 [thread overview]
Message-ID: <YW/q70dLyF+YudyF@T590> (raw)
In-Reply-To: <alpine.LSU.2.21.2110201014400.26817@pobox.suse.cz>
On Wed, Oct 20, 2021 at 10:19:27AM +0200, Miroslav Benes wrote:
> On Wed, 20 Oct 2021, Ming Lei wrote:
>
> > On Wed, Oct 20, 2021 at 08:43:37AM +0200, Miroslav Benes wrote:
> > > On Tue, 19 Oct 2021, Ming Lei wrote:
> > >
> > > > On Tue, Oct 19, 2021 at 08:23:51AM +0200, Miroslav Benes wrote:
> > > > > > > By you only addressing the deadlock as a requirement on approach a) you are
> > > > > > > forgetting that there *may* already be present drivers which *do* implement
> > > > > > > such patterns in the kernel. I worked on addressing the deadlock because
> > > > > > > I was informed livepatching *did* have that issue as well and so very
> > > > > > > likely a generic solution to the deadlock could be beneficial to other
> > > > > > > random drivers.
> > > > > >
> > > > > > In-tree zram doesn't have such deadlock, if livepatching has such AA deadlock,
> > > > > > just fixed it, and seems it has been fixed by 3ec24776bfd0.
> > > > >
> > > > > I would not call it a fix. It is a kind of ugly workaround because the
> > > > > generic infrastructure lacked (lacks) the proper support in my opinion.
> > > > > Luis is trying to fix that.
> > > >
> > > > What is the proper support of the generic infrastructure? I am not
> > > > familiar with livepatching's model(especially with module unload), you mean
> > > > livepatching have to do the following way from sysfs:
> > > >
> > > > 1) during module exit:
> > > >
> > > > mutex_lock(lp_lock);
> > > > kobject_put(lp_kobj);
> > > > mutex_unlock(lp_lock);
> > > >
> > > > 2) show()/store() method of attributes of lp_kobj
> > > >
> > > > mutex_lock(lp_lock)
> > > > ...
> > > > mutex_unlock(lp_lock)
> > >
> > > Yes, this was exactly the case. We then reworked it a lot (see
> > > 958ef1e39d24 ("livepatch: Simplify API by removing registration step"), so
> > > now the call sequence is different. kobject_put() is basically offloaded
> > > to a workqueue scheduled right from the store() method. Meaning that
> > > Luis's work would probably not help us currently, but on the other hand
> > > the issues with AA deadlock were one of the main drivers of the redesign
> > > (if I remember correctly). There were other reasons too as the changelog
> > > of the commit describes.
> > >
> > > So, from my perspective, if there was a way to easily synchronize between
> > > a data cleanup from module_exit callback and sysfs/kernfs operations, it
> > > could spare people many headaches.
> >
> > kobject_del() is supposed to do so, but you can't hold a shared lock
> > which is required in show()/store() method. Once kobject_del() returns,
> > no pending show()/store() any more.
> >
> > The question is that why one shared lock is required for livepatching to
> > delete the kobject. What are you protecting when you delete one kobject?
>
> I think it boils down to the fact that we embed kobject statically to
> structures which livepatch uses to maintain data. That is discouraged
> generally, but all the attempts to implement it correctly were utter
> failures.
OK, then it isn't one common usage, in which kobject covers the release
of the external object. What is the exact kobject in livepatching?
But kobject_del() won't release the kobject, you shouldn't need the lock
to delete kobject first. After the kobject is deleted, no any show() and
store() any more, isn't such sync[1] you expected?
Thanks,
Ming
next prev parent reply other threads:[~2021-10-20 10:10 UTC|newest]
Thread overview: 94+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-27 16:37 [PATCH v8 00/12] syfs: generic deadlock fix with module removal Luis Chamberlain
2021-09-27 16:37 ` [PATCH v8 01/12] LICENSES: Add the copyleft-next-0.3.1 license Luis Chamberlain
[not found] ` <202110050907.35FBD2A1@keescook>
[not found] ` <YWR2ZrtzChamY1y4@bombadil.infradead.org>
2021-10-11 17:57 ` Kees Cook
2021-09-27 16:37 ` [PATCH v8 02/12] testing: use the copyleft-next-0.3.1 SPDX tag Luis Chamberlain
2021-10-05 16:11 ` Kees Cook
2021-09-27 16:37 ` [PATCH v8 03/12] selftests: add tests_sysfs module Luis Chamberlain
2021-10-05 14:16 ` Greg KH
2021-10-05 16:57 ` Tim.Bird
2021-10-11 17:40 ` Luis Chamberlain
2021-10-11 17:38 ` Luis Chamberlain
2021-10-07 14:23 ` Miroslav Benes
2021-10-11 19:11 ` Luis Chamberlain
[not found] ` <202110050912.3DF681ED@keescook>
2021-10-11 19:03 ` Luis Chamberlain
2021-09-27 16:37 ` [PATCH v8 04/12] kernfs: add initial failure injection support Luis Chamberlain
2021-10-05 19:47 ` Kees Cook
2021-10-11 20:44 ` Luis Chamberlain
2021-09-27 16:37 ` [PATCH v8 05/12] test_sysfs: add support to use kernfs failure injection Luis Chamberlain
2021-10-05 19:51 ` Kees Cook
2021-10-11 20:56 ` Luis Chamberlain
2021-09-27 16:37 ` [PATCH v8 06/12] kernel/module: add documentation for try_module_get() Luis Chamberlain
2021-10-05 19:58 ` Kees Cook
2021-10-11 21:16 ` Luis Chamberlain
2021-09-27 16:38 ` [PATCH v8 07/12] fs/kernfs/symlink.c: replace S_IRWXUGO with 0777 on kernfs_create_link() Luis Chamberlain
2021-10-05 19:59 ` Kees Cook
2021-09-27 16:38 ` [PATCH v8 08/12] fs/sysfs/dir.c: replace S_IRWXU|S_IRUGO|S_IXUGO with 0755 sysfs_create_dir_ns() Luis Chamberlain
2021-10-05 16:05 ` Kees Cook
2021-09-27 16:38 ` [PATCH v8 09/12] sysfs: fix deadlock race with module removal Luis Chamberlain
2021-10-05 9:24 ` Ming Lei
2021-10-11 21:25 ` Luis Chamberlain
2021-10-12 0:20 ` Ming Lei
2021-10-12 21:18 ` Luis Chamberlain
2021-10-13 1:07 ` Ming Lei
2021-10-13 12:35 ` Luis Chamberlain
2021-10-13 15:04 ` Ming Lei
2021-10-13 21:16 ` Luis Chamberlain
2021-10-05 20:50 ` Kees Cook
2021-10-11 22:26 ` Luis Chamberlain
2021-10-13 12:41 ` Luis Chamberlain
2021-09-27 16:38 ` [PATCH v8 10/12] test_sysfs: enable deadlock tests by default Luis Chamberlain
2021-09-27 16:38 ` [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate Luis Chamberlain
2021-10-05 20:55 ` Kees Cook
2021-10-11 18:27 ` Luis Chamberlain
2021-10-14 1:55 ` Ming Lei
2021-10-14 2:11 ` Ming Lei
2021-10-14 20:24 ` Luis Chamberlain
2021-10-14 23:52 ` Ming Lei
2021-10-15 0:22 ` Luis Chamberlain
2021-10-15 8:36 ` Ming Lei
2021-10-15 8:52 ` Greg KH
2021-10-15 17:31 ` Luis Chamberlain
2021-10-16 11:28 ` Ming Lei
2021-10-18 19:32 ` Luis Chamberlain
2021-10-19 2:34 ` Ming Lei
2021-10-19 6:23 ` Miroslav Benes
2021-10-19 9:23 ` Ming Lei
2021-10-20 6:43 ` Miroslav Benes
2021-10-20 7:49 ` Ming Lei
2021-10-20 8:19 ` Miroslav Benes
2021-10-20 8:28 ` Greg KH
2021-10-25 9:58 ` Miroslav Benes
2021-10-20 10:09 ` Ming Lei [this message]
2021-10-26 8:48 ` Petr Mladek
2021-10-26 15:37 ` Ming Lei
2021-10-26 17:01 ` Luis Chamberlain
2021-10-27 11:57 ` Miroslav Benes
2021-10-27 14:27 ` Luis Chamberlain
2021-11-02 15:24 ` Petr Mladek
2021-11-02 16:25 ` Luis Chamberlain
2021-11-03 0:01 ` Ming Lei
2021-11-03 12:44 ` Luis Chamberlain
2021-10-27 11:42 ` Miroslav Benes
2021-11-02 14:15 ` Petr Mladek
2021-11-02 14:51 ` Petr Mladek
2021-11-02 15:17 ` Ming Lei
2021-11-02 14:56 ` Ming Lei
2021-10-19 15:28 ` Luis Chamberlain
2021-10-19 16:29 ` Ming Lei
2021-10-19 19:36 ` Luis Chamberlain
2021-10-20 1:15 ` Ming Lei
2021-10-20 15:48 ` Luis Chamberlain
2021-10-21 0:39 ` Ming Lei
2021-10-21 17:18 ` Luis Chamberlain
2021-10-22 0:05 ` Ming Lei
2021-10-19 15:50 ` Luis Chamberlain
2021-10-19 16:25 ` Greg KH
2021-10-19 16:30 ` Luis Chamberlain
2021-10-19 17:28 ` Greg KH
2021-10-19 19:46 ` Luis Chamberlain
2021-10-19 16:39 ` Ming Lei
2021-10-19 19:38 ` Luis Chamberlain
2021-10-20 0:55 ` Ming Lei
2021-09-27 16:38 ` [PATCH v8 12/12] zram: use ATTRIBUTE_GROUPS to fix sysfs deadlock module removal Luis Chamberlain
2021-10-05 20:57 ` Kees Cook
2021-10-11 18:28 ` Luis Chamberlain
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