centos man page for del_timer_sync

DEL_TIMER_SYNC(9)						   Driver Basics						 DEL_TIMER_SYNC(9)

NAME
       del_timer_sync - deactivate a timer and wait for the handler to finish.

SYNOPSIS
       int del_timer_sync(struct timer_list * timer);

ARGUMENTS
       timer
	   the timer to be deactivated

DESCRIPTION
       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.

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:

       CPU0 CPU1 ---- ---- <SOFTIRQ> call_timer_fn; base->running_timer = mytimer; spin_lock_irq(somelock); <IRQ> spin_lock(somelock);
       del_timer_sync(mytimer); 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 function returns whether it has deactivated a pending timer or not.

COPYRIGHT
Kernel Hackers Manual 3.10					     June 2014							 DEL_TIMER_SYNC(9)