cvs commit: src/sys/kern kern_fork.c

[David Xu]

Alfred Perlstein wrote:
> * David Xu <davidxu at FreeBSD.org> [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.
> 
thread_suspend_check() with SINGLE_BOUNDARY is used is only called
in userret() where I don't think any code is still using the p_fd.

Regards,
David Xu