Re: [PATCH v3 2/3] zram: fix deadlock with sysfs attribute usage and driver removal

From: Luis Chamberlain <mcgrof@kernel.org>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: minchan@kernel.org, jeyu@kernel.org, ngupta@vflare.org,
	sergey.senozhatsky.work@gmail.com, axboe@kernel.dk,
	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 v3 2/3] zram: fix deadlock with sysfs attribute usage and driver removal
Date: Tue, 22 Jun 2021 09:40:27 -0700	[thread overview]
Message-ID: <20210622164027.656zda4gjy2kjr5z@garbanzo> (raw)
In-Reply-To: <YNIPiDb3TPamz5cF@kroah.com>

On Tue, Jun 22, 2021 at 06:27:52PM +0200, Greg KH wrote:
> On Tue, Jun 22, 2021 at 08:27:13AM -0700, Luis Chamberlain wrote:
> > On Tue, Jun 22, 2021 at 09:41:23AM +0200, Greg KH wrote:
> > > On Mon, Jun 21, 2021 at 04:36:34PM -0700, Luis Chamberlain wrote:
> > > > +	ssize_t __ret; \
> > > > +	if (!try_module_get(THIS_MODULE)) \
> > > 
> > > try_module_get(THIS_MODULE) is always racy and probably does not do what
> > > you want it to do.  You always want to get/put module references from
> > > code that is NOT the code calling these functions.
> > 
> > In this case, we want it to trump module removal if it succeeds. That's all.
> 
> True, but either you stop the race, or you do not right?  If you are so
> invested in your load/unload test, this should show up with this code
> eventually as well.

I still do not see how the race is possible give the goal to prevent
module removal if a sysfs file is being used. If rmmod is taking
place, this simply will bail out.

> > > > +		return -ENODEV; \
> > > > +	__ret = _name ## _store(dev, attr, buf, len); \
> > > > +	module_put(THIS_MODULE); \
> > > 
> > > This too is going to be racy.
> > > 
> > > While fun to poke at, I still think this is pointless.
> > 
> > If you have a better idea, which does not "DOS" module removal, please
> > let me know!
> 
> I have yet to understand why you think that the load/unload in a loop is
> a valid use case.

That is dependent upon the intrastructure tests built for a driver.

In the case of fstests and blktests we have drivers which *always* get
removed and loaded on each test. Take for instance scsi_debug, which
creates / destroys virtual devices on the per test. Likewise, to build
confidence that failure rate is as close as possible to 0, one must run
a test as many times as possible in a loop. And, to build confidence in
a test, in some situations one ends up running modprobe / rmmod in a
loop.

In this case a customer does have a complex system of tests, and by looking
at the crash logs I managed to simplify the way to reproduce it using
simple shell scripts.

  Luis