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

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Message-ID: <20210622152713.fqwyuqpamwgaxomc@garbanzo>
Date:   Tue, 22 Jun 2021 08:27:13 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     minchan@...nel.org, jeyu@...nel.org, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, axboe@...nel.dk,
        mbenes@...e.com, jpoimboe@...hat.com, tglx@...utronix.de,
        keescook@...omium.org, jikos@...nel.org, rostedt@...dmis.org,
        peterz@...radead.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] zram: fix deadlock with sysfs attribute usage and
 driver removal

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.

> > +		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!

  Luis