del_timer_sync — deactivate a timer and wait for the handler to finish.
int del_timer_sync ( | struct timer_list * timer); |
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 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.
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.