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del_timer_sync(9) - phpMan

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DEL_TIMER_SYNC(9)                         Driver Basics                         DEL_TIMER_SYNC(9)

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

       int del_timer_sync(struct timer_list * 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.

       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.

       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.

Kernel Hackers Manual 4.8.                 January 2017                         DEL_TIMER_SYNC(9)

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