From: Greg KH <gregkh@linuxfoundation.org>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: akpm@linux-foundation.org, minchan@kernel.org, jeyu@kernel.org,
ngupta@vflare.org, sergey.senozhatsky.work@gmail.com,
rafael@kernel.org, axboe@kernel.dk, tj@kernel.org,
mbenes@suse.com, jpoimboe@redhat.com, tglx@linutronix.de,
keescook@chromium.org, jikos@kernel.org, rostedt@goodmis.org,
peterz@infradead.org, linux-block@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 2/3] zram: fix deadlock with sysfs attribute usage and module removal
Date: Fri, 23 Jul 2021 13:15:49 +0200 [thread overview]
Message-ID: <YPqk5WCBgvNQzq4S@kroah.com> (raw)
In-Reply-To: <20210722221705.kyrdkpt6fwf5k56o@garbanzo>
On Thu, Jul 22, 2021 at 03:17:05PM -0700, Luis Chamberlain wrote:
> On Wed, Jul 21, 2021 at 01:29:29PM +0200, Greg KH wrote:
> > On Fri, Jul 02, 2021 at 05:19:57PM -0700, Luis Chamberlain wrote:
> > > +#define MODULE_DEVICE_ATTR_FUNC_STORE(_name) \
> > > +static ssize_t module_ ## _name ## _store(struct device *dev, \
> > > + struct device_attribute *attr, \
> > > + const char *buf, size_t len) \
> > > +{ \
> > > + ssize_t __ret; \
> > > + if (!try_module_get(THIS_MODULE)) \
> > > + return -ENODEV; \
> >
> > I feel like this needs to be written down somewhere as I see it come up
> > all the time.
>
> I'll go ahead and cook up a patch to do just this after I send this
> email out.
>
> > Again, this is racy and broken code. You can NEVER try to increment
> > your own module reference count unless it has already been incremented
> > by someone external first.
>
> In the zram driver's case the sysfs files are still pegged on, because
> as we noted before the kernfs active reference will ensure the store
> operation still exists.
How does that happen without a module lock?
> If the driver removes the operation prior to
> getting the active reference, the write will just fail. kernfs ensures
> once a file is opened the op is not removed until the operation completes.
How does it do that?
> If a file is opened then, the module cannot possibly be removed. The
> piece of information we realy care about is the use of module_is_live()
> inside try_module_get() which does:
>
> static inline bool module_is_live(struct module *mod)
> {
> return mod->state != MODULE_STATE_GOING;
> }
>
> The try allows module removal to trump use of the sysfs file. If
> userspace wants the module removed, it gives up in favor for that
> operation.
I do not see the tie in kernfs to module reference counts, what am I
missing?
thanks,
greg k-h
next prev parent reply other threads:[~2021-07-23 11:15 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-03 0:19 [PATCH v6 0/3] zram: fix few sysfs races Luis Chamberlain
2021-07-03 0:19 ` [PATCH v6 1/3] zram: fix crashes with cpu hotplug multistate Luis Chamberlain
2021-07-03 0:19 ` [PATCH v6 2/3] zram: fix deadlock with sysfs attribute usage and module removal Luis Chamberlain
2021-07-10 19:28 ` Andrew Morton
2021-07-11 5:00 ` Greg KH
2021-07-12 23:17 ` Luis Chamberlain
2021-07-21 11:29 ` Greg KH
2021-07-22 22:17 ` Luis Chamberlain
2021-07-23 11:15 ` Greg KH [this message]
2021-07-23 17:49 ` Luis Chamberlain
2021-07-27 17:35 ` Luis Chamberlain
2021-07-03 0:19 ` [PATCH v6 3/3] zram: use ATTRIBUTE_GROUPS Luis Chamberlain
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