* 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.
*
在調用del_timer_sync這個函數時, 不能獲取在中斷上下文可能要佔用的鎖,不然會導致死鎖問題。 比如CPU0上已經獲取了somelock, 然後等待定時器的完成,此時CPU1進入軟中斷上下文時,會首先獲取somelock鎖,但由於該鎖已經被CPU0上的跑的代碼佔用了,會在CPU1上一直忙等待, 因此CPU0和CPU1的代碼都不會繼續跑下去了。