linux-kernel - Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex

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Message-ID: <81798fe5-f89e-482f-b0d0-674ccbfc3666@redhat.com>
Date:   Wed, 6 Dec 2023 16:01:01 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     George Stark <gnstark@...utedevices.com>, pavel@....cz,
        lee@...nel.org, vadimp@...dia.com, mpe@...erman.id.au,
        npiggin@...il.com, christophe.leroy@...roup.eu,
        mazziesaccount@...il.com, andy.shevchenko@...il.com,
        jic23@...nel.org
Cc:     linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kernel@...utedevices.com
Subject: Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex_init

Hi George,

On 12/4/23 19:05, George Stark wrote:
> Using of devm API leads to certain order of releasing resources.
> So all dependent resources which are not devm-wrapped should be deleted
> with respect to devm-release order. Mutex is one of such objects that
> often is bound to other resources and has no own devm wrapping.
> Since mutex_destroy() actually does nothing in non-debug builds
> frequently calling mutex_destroy() is just ignored which is safe for now
> but wrong formally and can lead to a problem if mutex_destroy() is
> extended so introduce devm_mutex_init().
> 
> Signed-off-by: George Stark <gnstark@...utedevices.com>
> ---
>  include/linux/devm-helpers.h | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/include/linux/devm-helpers.h b/include/linux/devm-helpers.h
> index 74891802200d..2f56e476776f 100644
> --- a/include/linux/devm-helpers.h
> +++ b/include/linux/devm-helpers.h
> @@ -76,4 +76,22 @@ static inline int devm_work_autocancel(struct device *dev,
>  	return devm_add_action(dev, devm_work_drop, w);
>  }
>  
> +static inline void devm_mutex_release(void *res)
> +{
> +	mutex_destroy(res);
> +}
> +
> +/**
> + * devm_mutex_init - Resource-managed mutex initialization
> + * @dev:	Device which lifetime work is bound to
> + * @lock:	Pointer to a mutex
> + *
> + * Initialize mutex which is automatically destroyed when driver is detached.
> + */
> +static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
> +{
> +	mutex_init(lock);
> +	return devm_add_action_or_reset(dev, devm_mutex_release, lock);
> +}
> +
>  #endif

mutex_destroy() only actually does anything if CONFIG_DEBUG_MUTEXES
is set, otherwise it is an empty inline-stub.

Adding a devres resource to the device just to call an empty inline
stub which is a no-op seems like a waste of resources. IMHO it
would be better to change this to:

static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
{
	mutex_init(lock);
#ifdef CONFIG_DEBUG_MUTEXES
	return devm_add_action_or_reset(dev, devm_mutex_release, lock);
#else
	return 0;
#endif
}

To avoid the unnecessary devres allocation when
CONFIG_DEBUG_MUTEXES is not set.

Regards,

Hans