From: Petr Mladek <pmladek@suse.com>
To: Miroslav Benes <mbenes@suse.cz>
Cc: Luis Chamberlain <mcgrof@kernel.org>,
Ming Lei <ming.lei@redhat.com>,
Julia Lawall <julia.lawall@inria.fr>,
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: Tue, 2 Nov 2021 16:24:06 +0100 [thread overview]
Message-ID: <YYFYFrnhwPiyOtst@alley> (raw)
In-Reply-To: <alpine.LSU.2.21.2110271343290.3655@pobox.suse.cz>
On Wed 2021-10-27 13:57:40, Miroslav Benes wrote:
> On Tue, 26 Oct 2021, Luis Chamberlain wrote:
>
> > On Tue, Oct 26, 2021 at 11:37:30PM +0800, Ming Lei wrote:
> > > On Tue, Oct 26, 2021 at 10:48:18AM +0200, Petr Mladek wrote:
> > > > Livepatch code never called kobject_del() under a lock. It would cause
> > > > the obvious deadlock.
I have to correct myself. IMHO, the deadlock is far from obvious. I
always get lost in the code and the documentation is not clear.
I always get lost.
> >
> > Never?
>
> kobject_put() to be precise.
IMHO, the problem is actually with kobject_del() that gets blocked
until the sysfs interface gets removed. kobject_put() will have
the same problem only when the clean up is not delayed.
> When I started working on the support for module/live patches removal,
> calling kobject_put() under our klp_mutex lock was the obvious first
> choice given how the code was structured, but I ran into problems with
> deadlocks immediately. So it was changed to async approach with the
> workqueue. Thus the mainline code has never suffered from this, but we
> knew about the issues.
>
> > > > The historic code only waited in the
> > > > module_exit() callback until the sysfs interface was removed.
> > >
> > > OK, then Luis shouldn't consider livepatching as one such issue to solve
> > > with one generic solution.
> >
> > It's not what I was told when the deadlock was found with zram, so I was
> > informed quite the contrary.
>
> >From my perspective, it is quite easy to get it wrong due to either a lack
> of generic support, or missing rules/documentation. So if this thread
> leads to "do not share locks between a module removal and a sysfs
> operation" strict rule, it would be at least something. In the same
> manner as Luis proposed to document try_module_get() expectations.
The rule "do not share locks between a module removal and a sysfs
operation" is not clear to me.
IMHO, there are the following rules:
1. rule: kobject_del() or kobject_put() must not be called under a lock that
is used by store()/show() callbacks.
reason: kobject_del() waits until the sysfs interface is destroyed.
It has to wait until all store()/show() callbacks are finished.
2. rule: kobject_del()/kobject_put() must not be called from the
related store() callbacks.
reason: same as in 1st rule.
3. rule: module_exit() must wait until all release() callbacks are called
when kobject are static.
reason: kobject_put() must be called to clean up internal
dependencies. The clean up might be done asynchronously
and need access to the kobject structure.
Best Regards,
Petr
PS: I am sorry if I am messing things. I want to be sure that we are
all talking about the same and understand it the same way.
next prev parent reply other threads:[~2021-11-02 15:24 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <YWeR4moCRh+ZHOmH@T590>
[not found] ` <YWiSAN6xfYcUDJCb@bombadil.infradead.org>
[not found] ` <YWjCpLUNPF3s4P2U@T590>
[not found] ` <YWjJ0O7K+31Iz3ox@bombadil.infradead.org>
[not found] ` <YWk9e957Hb+I7HvR@T590>
[not found] ` <YWm68xUnAofop3PZ@bombadil.infradead.org>
[not found] ` <YWq3Z++uoJ/kcp+3@T590>
[not found] ` <YW3LuzaPhW96jSBK@bombadil.infradead.org>
[not found] ` <YW4uwep3BCe9Vxq8@T590>
[not found] ` <alpine.LSU.2.21.2110190820590.15009@pobox.suse.cz>
[not found] ` <YW6OptglA6UykZg/@T590>
2021-10-20 6:43 ` [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate 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
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 [this message]
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
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