linux-kernel - Re: [PATCH v2 4/4] test_sysfs: demonstrate deadlock fix

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Date:   Sat, 3 Jul 2021 06:49:46 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     tj@...nel.org, shuah@...nel.org, akpm@...ux-foundation.org,
        rafael@...nel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, andriin@...com, daniel@...earbox.net,
        atenart@...nel.org, alobakin@...me, weiwan@...gle.com,
        ap420073@...il.com, jeyu@...nel.org, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, minchan@...nel.org,
        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, netdev@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] test_sysfs: demonstrate deadlock fix

On Fri, Jul 02, 2021 at 05:46:32PM -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; \
> +	__ret = _name ## _store(dev, attr, buf, len); \
> +	module_put(THIS_MODULE); \
> +	return __ret; \
> +}

As I have pointed out before, doing try_module_get(THIS_MODULE) is racy
and should not be added back to the kernel tree.  We got rid of many
instances of this "bad pattern" over the years, please do not encourage
it to be added back as others will somehow think that it correct code.

I'll go over the rest of this after 5.14-rc1 is out, am busy until then.

thanks,

greg k-h