New "timeout" api, to replace callout
andre at freebsd.org
Wed Jan 2 15:20:41 PST 2008
John Baldwin wrote:
> On Sunday 02 December 2007 07:53:18 am Andre Oppermann wrote:
>> Poul-Henning Kamp wrote:
>>> In message <4752998A.9030007 at freebsd.org>, Andre Oppermann writes:
>>>> o TCP puts the timer into an allocated structure and upon close of the
>>>> session it has to be deallocated including stopping of all currently
>>>> running timers.
>>>> -> The timer facility should provide an atomic stop/remove call
>>>> that prevent any further callbacks upon return. It should not
>>>> do a 'drain' where the callback may be run anyway.
>>>> Note: We hold the lock the callback would have to obtain.
>>> It is my intent, that the implementation behind the new API will
>>> only ever grab the specified lock when it calls the timeout function.
>> This is the same for the current one and pretty much a given.
>>> When you do a timeout_disable() or timeout_cleanup() you will be
>>> sleeping on a mutex internal to the implementation, if the timeout
>>> is currently executing.
>> This is the problematic part. We can't sleep in TCP when cleaning up
>> the timer. We're not always called from userland but from interrupt
>> context. And when calling the cleanup we currently hold the lock the
>> callout wants to obtain. We can't drop it either as the race would
>> be back again. What you describe here is the equivalent of callout_
>> drain(). This is unfortunately unworkable in TCP's context. The
>> callout has to go away even if it is already pending and waiting on
>> the lock. Maybe that can only be solved by a flag in the lock saying
>> "give up and go away".
> The reason you need to do a drain is to allow for safe destroying of the lock.
> Specifically, drivers tend to do this:
> If you don't have the drain and softclock is trying to acquire the backing
> mutex while you have it held (before the callout_stop) then Bad Things can
> happen if you don't do the drain. Having the lock just "give up" doesn't
> work either because if the memory containing the lock is free'd and
> reinitialized such that it looks enough like a valid lock then softclock (or
> its equivalent) will still try to obtain it. Also, you need to do a drain so
> it is safe to free the callout structure to prevent it from being recycled
> and having weird races where it gets recycled and rescheduled but the timer
> code thinks it has a pending stop for that pointer and so it aborts the wrong
> instance of the timer, etc.
This is all well known. ;) What isn't known is that this (the
sleep part) is a major problem for TCP due to being run from
interrupt context. Hence the request for some kind of busy-drain
or other method prevent the sleep. A second less severe problem
are races while the lock is dropped during the sleep. Here some
other part of TCP may go into the tcpcb scheduled for destruction.
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