linux-kernel - Re: [PATCH v3 03/11] devm-helpers: introduce devm_mutex

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Date:   Thu, 14 Dec 2023 10:06:15 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     George Stark <gnstark@...utedevices.com>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "pavel@....cz" <pavel@....cz>, "lee@...nel.org" <lee@...nel.org>,
        "vadimp@...dia.com" <vadimp@...dia.com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "hdegoede@...hat.com" <hdegoede@...hat.com>,
        "mazziesaccount@...il.com" <mazziesaccount@...il.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "will@...nel.org" <will@...nel.org>,
        "longman@...hat.com" <longman@...hat.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "nikitos.tr@...il.com" <nikitos.tr@...il.com>
CC:     "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "kernel@...utedevices.com" <kernel@...utedevices.com>
Subject: Re: [PATCH v3 03/11] devm-helpers: introduce devm_mutex_init



Le 13/12/2023 à 23:30, George Stark a écrit :
> [Vous ne recevez pas souvent de courriers de gnstark@...utedevices.com. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
> 
> Using of devm API leads to a 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 wrapper.
> 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().

So you abandonned the idea of using mutex.h ?

I can't see the point to spread mutex functions into devm-helpers.h

Adding a mutex_destroy macro for this purpose looks odd. And if someone 
defines a new version of mutex_destroy() and forget the macro, it will 
go undetected.

Usually macros of that type serve the purpose of defining a fallback 
when the macro is not defined. In that case, when someone adds a new 
version without defining the macro, it gets detected because if 
conflicts with the fallback.
But in your case it works the other way round, so I will just go undetected.

For me the best solution remains to use mutex.h and have 
devm_mutex_init() defined or declared at the same place as mutex_destroy().


> 
> Signed-off-by: George Stark <gnstark@...utedevices.com>
> ---
>   include/linux/devm-helpers.h | 27 +++++++++++++++++++++++++++
>   1 file changed, 27 insertions(+)
> 
> diff --git a/include/linux/devm-helpers.h b/include/linux/devm-helpers.h
> index 74891802200d..4043c3481d2e 100644
> --- a/include/linux/devm-helpers.h
> +++ b/include/linux/devm-helpers.h
> @@ -24,6 +24,7 @@
>    */
> 
>   #include <linux/device.h>
> +#include <linux/mutex.h>
>   #include <linux/workqueue.h>
> 
>   static inline void devm_delayed_work_drop(void *res)
> @@ -76,4 +77,30 @@ static inline int devm_work_autocancel(struct device *dev,
>          return devm_add_action(dev, devm_work_drop, w);
>   }
> 
> +#ifdef mutex_destroy
> +static inline void devm_mutex_release(void *res)
> +{
> +       mutex_destroy(res);
> +}
> +#endif
> +
> +/**
> + * devm_mutex_init - Resource-managed mutex initialization
> + * @dev:       Device which lifetime mutex is bound to
> + * @lock:      Pointer to a mutex
> + *
> + * Initialize mutex which is automatically destroyed when the driver is detached.
> + *
> + * Returns: 0 on success or a negative error code on failure.
> + */
> +static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
> +{
> +       mutex_init(lock);
> +#ifdef mutex_destroy
> +       return devm_add_action_or_reset(dev, devm_mutex_release, lock);
> +#else
> +       return 0;
> +#endif
> +}
> +
>   #endif
> --
> 2.25.1
>