Message-ID: <20221115202117.323694948@linutronix.de>
Date:   Tue, 15 Nov 2022 21:28:43 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...uxfoundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Julia Lawall <Julia.Lawall@...ia.fr>,
        Arnd Bergmann <arnd@...db.de>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Marc Zyngier <maz@...nel.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        linux-bluetooth@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: [patch 06/15] timers: Update kernel-doc for various functions

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

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/timer.c |  131 ++++++++++++++++++++++++++++++----------------------
 1 file changed, 77 insertions(+), 54 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 - 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.
+ * mod_timer_pending() is the same for pending timers as mod_timer(), but
+ * will not activate inactive timers.
  *
- * It is useful for unserialized use of 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)
 {
@@ -1137,9 +1139,9 @@ 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 - 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)
@@ -1152,9 +1154,11 @@ EXPORT_SYMBOL(mod_timer_pending);
  * 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)
 {
@@ -1168,8 +1172,15 @@ EXPORT_SYMBOL(mod_timer);
  * @expires:	New 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 +1189,18 @@ 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
+ * 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 +1211,12 @@ 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 +1252,17 @@ 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.)
+ * Contrary to del_timer_sync() this function does not wait for an
+ * eventually running timer callback on a different CPU and 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 +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.
+ *
+ * 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 +1389,19 @@ 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 +1415,17 @@ 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)
 {

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