Man page 9 : del_timer_sync

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

int del_timer_sync(struct timer_list *  timer );

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.

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 timerAqs handler. The timerAqs 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, 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. HereAqs 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.