[patch V2 07/17] timers: Update kernel-doc for various functions

Thomas Gleixner
2022-11-22T17:45:46Z

The kernel-doc of timer related functions is partially uncomprehensible
word salad. Rewrite it to make it useful.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
---
V2: Refined comments (Steven)
---
kernel/time/timer.c | 148 ++++++++++++++++++++++++++++++----------------------
1 file changed, 88 insertions(+), 60 deletions(-)

--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1121,14 +1121,16 @@ static inline int
}

/**
- * mod_timer_pending - modify a pending timer's timeout
- * @timer: the pending timer to be modified
- * @expires: new timeout in jiffies
- *
- * mod_timer_pending() is the same for pending timers as mod_timer(),
- * but will not re-activate and modify already deleted timers.
- *
- * It is useful for unserialized use of timers.
+ * mod_timer_pending - Modify a pending timer's timeout
+ * @timer: The pending timer to be modified
+ * @expires: New absolute timeout in jiffies
+ *
+ * mod_timer_pending() is the same for pending timers as mod_timer(), but
+ * will not activate inactive timers.
+ *
+ * Return:
+ * * %0 - The timer was inactive and not modified
+ * * %1 - The timer was active and requeued to expire at @expires
*/
int mod_timer_pending(struct timer_list *timer, unsigned long expires)
{
@@ -1137,24 +1139,27 @@ int mod_timer_pending(struct timer_list
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() is a more efficient way to update the expire field of an
- * active timer (if the timer is inactive it will be activated)
+ * mod_timer - Modify a timer's timeout
+ * @timer: The timer to be modified
+ * @expires: New absolute timeout in jiffies
*
* mod_timer(timer, expires) is equivalent to:
*
* del_timer(timer); timer->expires = expires; add_timer(timer);
*
+ * mod_timer() is more efficient than the above open coded sequence. In
+ * case that the timer is inactive, the del_timer() part is a NOP. The
+ * timer is in any case activated with the new expiry time @expires.
+ *
* Note that if there are multiple unserialized concurrent users of the
* same timer, then mod_timer() is the only safe way to modify the timeout,
* since add_timer() cannot modify an already running timer.
*
- * The function returns whether it has modified a pending timer or not.
- * (ie. mod_timer() of an inactive timer returns 0, mod_timer() of an
- * active timer returns 1.)
+ * Return:
+ * * %0 - The timer was inactive and started
+ * * %1 - The timer was active and requeued to expire at @expires or
+ * the timer was active and not modified because @expires did
+ * not change the effective expiry time
*/
int mod_timer(struct timer_list *timer, unsigned long expires)
{
@@ -1165,11 +1170,18 @@ EXPORT_SYMBOL(mod_timer);
/**
* timer_reduce - Modify a timer's timeout if it would reduce the timeout
* @timer: The timer to be modified
- * @expires: New timeout in jiffies
+ * @expires: New absolute timeout in jiffies
*
* timer_reduce() is very similar to mod_timer(), except that it will only
- * modify a running timer if that would reduce the expiration time (it will
- * start a timer that isn't running).
+ * modify an enqueued timer if that would reduce the expiration time. If
+ * @timer is not enqueued it starts the timer.
+ *
+ * Return:
+ * * %0 - The timer was inactive and started
+ * * %1 - The timer was active and requeued to expire at @expires or
+ * the timer was active and not modified because @expires
+ * did not change the effective expiry time such that the
+ * timer would expire earlier than already scheduled
*/
int timer_reduce(struct timer_list *timer, unsigned long expires)
{
@@ -1178,18 +1190,18 @@ int timer_reduce(struct timer_list *time
EXPORT_SYMBOL(timer_reduce);

/**
- * add_timer - start a timer
- * @timer: the timer to be added
+ * add_timer - Start a timer
+ * @timer: The timer to be started
*
- * The kernel will do a ->function(@timer) callback from the
- * timer interrupt at the ->expires point in the future. The
- * current time is 'jiffies'.
+ * Start @timer to expire at @timer->expires in the future. @timer->expires
+ * is the absolute expiry time measured in 'jiffies'. When the timer expires
+ * timer->function(timer) will be invoked from soft interrupt context.
*
- * The timer's ->expires, ->function fields must be set prior calling this
- * function.
+ * The @timer->expires and @timer->function fields must be set prior
+ * to calling this function.
*
- * Timers with an ->expires field in the past will be executed in the next
- * timer tick.
+ * If @timer->expires is already in the past @timer will be queued to
+ * expire at the next timer tick.
*/
void add_timer(struct timer_list *timer)
{
@@ -1200,11 +1212,12 @@ void add_timer(struct timer_list *timer)
EXPORT_SYMBOL(add_timer);

/**
- * add_timer_on - start a timer on a particular CPU
- * @timer: the timer to be added
- * @cpu: the CPU to start it on
+ * add_timer_on - Start a timer on a particular CPU
+ * @timer: The timer to be started
+ * @cpu: The CPU to start it on
*
- * This is not very scalable on SMP. Double adds are not possible.
+ * This can only operate on an inactive timer. Attempts to invoke this on
+ * an active timer are rejected with a warning.
*/
void add_timer_on(struct timer_list *timer, int cpu)
{
@@ -1240,15 +1253,18 @@ void add_timer_on(struct timer_list *tim
EXPORT_SYMBOL_GPL(add_timer_on);

/**
- * del_timer - deactivate a timer.
- * @timer: the timer to be deactivated
- *
- * del_timer() deactivates a timer - this works on both active and inactive
- * timers.
+ * del_timer - Deactivate a timer.
+ * @timer: The timer to be deactivated
*
- * 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.)
+ * 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.
+ * It neither 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 +1286,19 @@ 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 tries to deactivate a timer. On success the timer is not
+ * queued and the timer callback function is not running on any CPU.
+ *
+ * This function does not guarantee that the timer cannot be rearmed right
+ * after dropping the base lock. That needs to be prevented by the calling
+ * code if necessary.
+ *
+ * Return:
+ * * %0 - The timer was not pending
+ * * %1 - The timer was pending and deactivated
+ * * %-1 - The timer callback function is running on a different CPU
*/
int try_to_del_timer_sync(struct timer_list *timer)
{
@@ -1369,23 +1394,19 @@ static inline void del_timer_wait_runnin

#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
/**
- * del_timer_sync - deactivate a timer and wait for the handler to finish.
- * @timer: the timer to be deactivated
- *
- * This function only differs from del_timer() on SMP: besides deactivating
- * the timer it also makes sure the handler has finished executing on other
- * CPUs.
+ * del_timer_sync - Deactivate a timer and wait for the handler to finish.
+ * @timer: The timer to be deactivated
*
* Synchronization rules: Callers must prevent restarting of the timer,
* otherwise this function is meaningless. It must not be called from
* interrupt contexts unless the timer is an irqsafe one. The caller must
- * not hold locks which would prevent completion of the timer's
- * handler. The timer's handler must not call add_timer_on(). Upon exit the
- * timer is not queued and the handler is not running on any CPU.
- *
- * Note: For !irqsafe timers, you must not hold locks that are held in
- * interrupt context while calling this function. Even if the lock has
- * nothing to do with the timer in question. Here's why::
+ * not hold locks which would prevent completion of the timer's callback
+ * function. The timer's handler must not call add_timer_on(). Upon exit
+ * the timer is not queued and the handler is not running on any CPU.
+ *
+ * For !irqsafe timers, the caller must not hold locks that are held in
+ * interrupt context. Even if the lock has nothing to do with the timer in
+ * question. Here's why::
*
* CPU0 CPU1
* ---- ----
@@ -1399,10 +1420,17 @@ static inline void del_timer_wait_runnin
* while (base->running_timer == mytimer);
*
* Now del_timer_sync() will never return and never release somelock.
- * The interrupt on the other CPU is waiting to grab somelock but
- * it has interrupted the softirq that CPU0 is waiting to finish.
+ * The interrupt on the other CPU is waiting to grab somelock but it has
+ * interrupted the softirq that CPU0 is waiting to finish.
*
- * The function returns whether it has deactivated a pending timer or not.
+ * This function cannot guarantee that the timer is not rearmed again by
+ * some concurrent or preempting code, right after it dropped the base
+ * lock. If there is the possibility of a concurrent rearm then the return
+ * value of the function is meaningless.
+ *
+ * Return:
+ * * %0 - The timer was not pending
+ * * %1 - The timer was pending and deactivated
*/
int del_timer_sync(struct timer_list *timer)
{