[M3devel] fork/cvsup

[Tony Hosking]

Why not just exec in the child?

On 17 Mar 2010, at 03:47, Jay K wrote:

> http://developer.apple.com/mac/library/documentation/Darwin/Reference/ManPages/man2/fork.2.html
>  
>  
> There are limits to what you can do in the child process.  To be totally safe you should restrict your-self yourself
>      self to only executing async-signal safe operations until such time as one of the exec functions is
>      called.  All APIs, including global data symbols, in any framework or library should be assumed to be
>      unsafe after a fork() unless explicitly documented to be safe or async-signal safe.  If you need to use
>      these frameworks in the child process, you must exec.  In this situation it is reasonable to exec your-self. yourself.
>      self.
> 
>  
> http://www.opengroup.org/onlinepubs/000095399/functions/fork.html
>  
> Consequently, to avoid errors, the child process may only execute async-signal-safe operations until such time as one of theexec functions is called. [THR]   Fork handlers may be established by means of the pthread_atfork() function in order to maintain application invariants across fork() calls. 
>  
>  
> I've run through a few theories so far.
> Current thinking is related to what Tony said:
>  use pthread_atfork: 
>    in prepare, stopworld 
>    in parent, resumeworld 
>    You don't want the child to be mid-gc for example, on another thread. Or mid-anything.
>    in child, reinitialize -- current thread is the only thread
>  
>  
> Also in the cvsup code, ShutDown should just call DoShutDown immediately.
> I did that, without m3core changes, and it hits an error in the pthread code, signaling a nonexistant thread.
> pthread_atfork/child should address that -- child shouldn't retain a record of all the threads in the parent.
>  
>  
> I don't have a theory as to why user threads work.
>  
>  
> I experimented with malloc vs. static alloc vs. sbrk vs. mmap(private) vs. mmap(shared).
> I was expecting more cases to act like mmap(shared), but none did, only it.
>  
>  
> I experimented with having mutexes and condition variables be initialize up front instead of on-demand.
> Via changing cvsup to lock/unlock or broadcast immediately upon creating them.
> On the theory that might let them work across process.
> That didn't make a difference.
>  
>  
>  - Jay

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