On 15 Aug 2013, at 09:22, liu ping fan wrote: > How about new_list for vcpu to add timer, an before walking, splice > the new_list to timer_list?
If I understand you right, you would have to be careful any timer routine modified itself or (less likely) any other timer, as that timer would no longer be on the timer list, but rather be on the 'walk list'. There's no problem per se with them being on a new timer list, but some of them may modify the existing timer leaving it on the list you are walking, and some might put it on the timerlist associated with the clock. I'm not sure things expect timers to move between lists. Whatever, that means you can't call the callback with the mutex held in either the walk list or the timer list. I believe it's reasonably common for timers to call timer_mod on themselves to get them to fire again. The best I can do is simply take the lock, examine the timer at the head of the list (always), remove the timer from the active list, unlock, and call the callback. That's exactly what the patch does. (side note: you can't just splice the lists because they have to be ordered) -- Alex Bligh
