Subject: Re: [patch 06/15] timers: Update kernel-doc for various functions

From: Thomas Gleixner <[email-masked]>
Date: Tue, 22 Nov 2022 16:18:37 +0100

On Mon, Nov 21 2022 at 15:43, Steven Rostedt wrote:
> On Tue, 15 Nov 2022 21:28:43 +0100 (CET)
> Thomas Gleixner <[email-masked]> wrote:
>>  EXPORT_SYMBOL(mod_timer_pending);
>>  
>>  /**
>> - * mod_timer - modify a timer's timeout
>> - * @timer: the timer to be modified
>> - * @expires: new timeout in jiffies
>> + * mod_timer - Modify a timer's timeout
>> + * @timer:	The timer to be modified
>> + * @expires:	New timeout in jiffies
>>   *
>>   * mod_timer() is a more efficient way to update the expire field of an
>
> BTW, one can ask, "more efficient" than what?
>
> If you are updating this, perhaps swap it around a little.
>
>  * mod_timer(timer, expires) is equivalent to:
>  *
>  *     del_timer(timer); timer->expires = expires; add_timer(timer);
>  *
>  * mod_timer() is a more efficient way to update the expire field of an
>  * active timer (if the timer is inactive it will be activated)
>  *
>
> As seeing the equivalent first and then seeing "more efficient" makes a bit
> more sense.

Point taken.

>>   *
>> - * The timer's ->expires, ->function fields must be set prior calling this
>> - * function.
>> + * The @timer->expires and @timer->function fields must be set prior
>> + * calling this function.
>
>  "set prior to calling this function"

Fixed.

>>   *
>> - * The function returns whether it has deactivated a pending timer or not.
>> - * (ie. del_timer() of an inactive timer returns 0, del_timer() of an
>> - * active timer returns 1.)
>> + * Contrary to del_timer_sync() this function does not wait for an
>> + * eventually running timer callback on a different CPU and it neither
>
> I'm a little confused with the "eventually running timer". Does that simply
> mean one that is about to run next (that is, it doesn't handle race
> conditions and the timer is in the process of starting), but will still
> deactivate one that has not been started and the timer code for that CPU
> hasn't triggered yet?

Let me try again.

  The function only deactivates a pending timer, but contrary to
  del_timer_sync() it does not take into account whether the timers
  callback function is concurrently executed on a different CPU or not.

Does that make more sense?

>> + * prevents rearming of the timer.  If @timer can be rearmed concurrently
>> + * then the return value of this function is meaningless.
>> + *
>> + * Return:
>> + * * %0 - The timer was not pending
>> + * * %1 - The timer was pending and deactivated
>>   */
>>  int del_timer(struct timer_list *timer)
>>  {
>> @@ -1270,10 +1284,16 @@ EXPORT_SYMBOL(del_timer);
>>  
>>  /**
>>   * try_to_del_timer_sync - Try to deactivate a timer
>> - * @timer: timer to delete
>> + * @timer:	Timer to deactivate
>>   *
>> - * This function tries to deactivate a timer. Upon successful (ret >= 0)
>> - * exit the timer is not queued and the handler is not running on any CPU.
>> + * This function cannot guarantee that the timer cannot be rearmed right
>> + * after dropping the base lock. That needs to be prevented by the calling
>> + * code if necessary.
>
>
> Hmm, you seemed to have deleted the description of what the function does
> and replaced it with only what it cannot do.

Ooops.