linux-kernel - Re: [PATCH 2/9] timers: provide a "modern" variant of timers

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Date:   Tue, 16 May 2017 22:03:56 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Mark Gross <mark.gross@...el.com>, Tejun Heo <tj@...nel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] timers: provide a "modern" variant of timers

On Tue, May 16, 2017 at 1:48 PM, Christoph Hellwig <hch@....de> wrote:

>         unsigned long           expires;
> -       void                    (*function)(unsigned long);
> +       union {
> +               void            (*func)(struct timer_list *timer);
> +               void            (*function)(unsigned long);
> +       };
...
> +#define INIT_TIMER(_func, _expires, _flags)            \
> +{                                                      \
> +       .entry = { .next = TIMER_ENTRY_STATIC },        \
> +       .func = (_func),                                \
> +       .expires = (_expires),                          \
> +       .flags = TIMER_MODERN | (_flags),               \
> +       __TIMER_LOCKDEP_MAP_INITIALIZER(__FILE__ ":" __stringify(__LINE__)) \
> +}

If I remember correctly, this will fail with gcc-4.5 and earlier, which can't
use named initializers for anonymous unions. One of these two should
work, but they are both ugly:

a) don't use a named initializer for the union (a bit fragile)

 +#define INIT_TIMER(_func, _expires, _flags)            \
 +{                                                      \
 +       .entry = { .next = TIMER_ENTRY_STATIC },        \
 +       .expires = (_expires),                          \
 +       { .func = (_func) },                                \
 +       .flags = TIMER_MODERN | (_flags),               \
 +       __TIMER_LOCKDEP_MAP_INITIALIZER(__FILE__ ":" __stringify(__LINE__)) \
 +}

b) give the union a name (breaks any reference to timer_list->func in C code):

 +       union {
 +               void            (*func)(struct timer_list *timer);
 +               void            (*function)(unsigned long);
 +       } u;
...
 +#define INIT_TIMER(_func, _expires, _flags)            \
 +{                                                      \
 +       .entry = { .next = TIMER_ENTRY_STATIC },        \
 +       .u.func = (_func),                                \
 +       .expires = (_expires),                          \
 +       .flags = TIMER_MODERN | (_flags),               \
 +       __TIMER_LOCKDEP_MAP_INITIALIZER(__FILE__ ":" __stringify(__LINE__)) \
 +}

> +/**
> + * prepare_timer - initialize a timer before first use
> + * @timer:     timer structure to prepare
> + * @func:      callback to be called when the timer expires
> + * @flags      %TIMER_* flags that control timer behavior
> + *
> + * This function initializes a timer_list structure so that it can
> + * be used (by calling add_timer() or mod_timer()).
> + */
> +static inline void prepare_timer(struct timer_list *timer,
> +               void (*func)(struct timer_list *timer), u32 flags)
> +{
> +       __init_timer(timer, TIMER_MODERN | flags);
> +       timer->func = func;
> +}
> +
> +static inline void prepare_timer_on_stack(struct timer_list *timer,
> +               void (*func)(struct timer_list *timer), u32 flags)
> +{
> +       __init_timer_on_stack(timer, TIMER_MODERN | flags);
> +       timer->func = func;
> +}

I fear this breaks lockdep output, which turns the name of
the timer into a string that gets printed later. It should work
when these are macros, or a macro wrapping an inline function
like __init_timer is.

      Arnd