Re: New "timeout" api, to replace callout

John Baldwin wrote:
On Sunday 02 December 2007 07:53:18 am Andre Oppermann wrote:
Poul-Henning Kamp wrote:
In message <4752998A.9030007@xxxxxxxxxxx>, 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:

FOO_LOCK(sc);
...
callout_stop(...);
FOO_UNLOCK(sc);
...
callout_drain(...);
...
mtx_destroy(&sc->foo_mtx);

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.

--
Andre

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Relevant Pages

  • Re: New "timeout" api, to replace callout
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  • Re: New "timeout" api, to replace callout
    ... -> The timer facility should provide an atomic stop/remove call ... We hold the lock the callback would have to obtain. ... What you describe here is the equivalent of callout_ ...
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  • Re: New "timeout" api, to replace callout
    ... -> The timer facility should provide an atomic stop/remove call ... do a 'drain' where the callback may be run anyway. ... We hold the lock the callback would have to obtain. ... What you describe here is the equivalent of callout_ ...
    (freebsd-arch)
  • Re: New "timeout" api, to replace callout
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  • Re: New "timeout" api, to replace callout
    ... If you don't have the drain and softclock is trying to acquire the backing mutex while you have it held 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 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. ... The only difference is that I pass an error code to the callback which might happen after that usbd_transfer_stop is called. ...
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