New "timeout" api, to replace callout

[Andre Oppermann]

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".

-- 
Andre