cvs commit: src/sys/kern kern_fork.c

Julian Elischer julian at
Wed Oct 24 22:23:03 PDT 2007

Alfred Perlstein wrote:
> * David Xu <davidxu at> [071024 18:34] wrote:
>> Julian Elischer wrote:
>>> julian      2007-10-23 17:54:16 UTC
>>>  FreeBSD src repository
>>>  Modified files:
>>>    sys/kern             kern_fork.c 
>>>  Log:
>>>  Take out the single-threading code in fork.
>>>  After discussions with jeff, alc, (various Ironport people), david Xu,
>>>  and mostly Alfred (who found the problem) it has been demonstrated that 
>>>  this
>>>  is not needed for our implementations of threads and represents a real
>>>  (as in we've seen it happen a lot) deadlock danger.
>>> ...
>> I think if process is forking a thread, that says flag RFPROC is not
>> set and flags RFCFDG or RFCFDG is set, you still need to call
>> thread_single(SINGLE_BOUNDARY), otherwise, for a threaded process,
>> the memory pointed by p_fd is freed while other threads are using it,
>> it will cause kernel to panic.
> This is unlikely to be fixed by SINGLE_BOUNDARY and will likely require
> refcounting to fix.  SINGLE_BOUNDARY will not fix the locations where
> this happens:
>         p = td->td_proc;
>         fdp = p->p_fd;
>         do something that blocks...
>         re-use fdp.

the SINGLE_BOUNDARY wouldn't let this happen, as all threads would have to be at the
user boundary to be conformant and they can not have such references there.
I'm not convinced however that it is fully functional at this time however.
A quick read-through to see what it was doing gave me a headache.

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