[M3devel] fork/cvsup

Wed Mar 17 19:30:47 CET 2010

I don't think there is a "general" solution to this that should be applied to the thread library.  Modula-3 does not mandate any support for fork!  It is not part of the language.  If a program relies on platform-specific interfaces then it must be the one to handle situations arising from the problem.  Why does cvsup need to fork in the first place?  Surely it can simply add threads to handle clients as they arrive?

Antony Hosking | Associate Professor | Computer Science | Purdue University
305 N. University Street | West Lafayette | IN 47907 | USA
Office +1 765 494 6001 | Mobile +1 765 427 5484

On 17 Mar 2010, at 14:13, Jay K wrote:

> ---
> bad news:
> It doesn't completely work. It works a bunch of times in a row, like 9, then hangs.
> Restart manually. Works again. Around 9 times. Then hangs again.
> That is on Linux/x86 and Solaris/sparc.
> Doesn't work at all on Mac/amd64, just hangs.
>  
> ---
> sketch:
> m3core uses pthread_atfork to selectively reinitialize
>   Mainly to only have one thread.
>  
>  
> common Thread.PThreadAtFork is provided for others to do the same
>   It is deliberately in a portable interface.
>  
>  
> Thread.ReforkThreadAfterProcessFork
>   Is provided for users to restart threads from their child AtFork hander.
>   This is used by the allocator/collector.
> 
>  
> Thread.ForkProcessAndAllThreads()
>   Is used by "lazy" clients who want to restart all their threads
>   but didn't keep track of them. The runtime can do it for them.
>  
>  
> This allows for "fork + do work" folks do call or not call ForkProcessAndAllThreads
> or not, depending on if they need their threads restarted.
> The runtime takes care of its threads either way.
>  
>  
> ---
> What'd I'd written up:
>  
> attached works typically 9 times on Linux and Solaris
> before server hangs again.
> 
>  
> No improvement on Darwin, just hangs.
> Can't see much in debuggers for some reason.
>  
> 
> There is extra allowance in the m3core change such
>  that users of fork + do work (as opposed to fork + exec)
>  may or may not call ForkAll, depending on if they
>  feel a need for their own threads to be recreated,
>  and if they've kept track of how to recreate them,
>  or just rely on the runtime to know all the threads.
>  
> 
> There are three runtime threads that are sometimes
> created in the parent, and if so, recreated in the child.
> background collector, foreground collector, weak ref thread
>  
>  
> I'll try to poke at it some more.
>  
> 
> I'm not sure what is the best way to suspend all threads.
> I tried a few differnt ways.
>   SuspendOthers
>   LockHeap
>   pthread_mutex_lock
>   various combinations
> 
>  
> It is deliberate that pthread specific code is in common/Thread.i3.
> That way code can be portable, at least among the two Posix thread implementations.
>  
> 
>  - Jay
> 
> 
>  
> From: hosking at cs.purdue.edu
> Date: Wed, 17 Mar 2010 14:01:31 -0400
> To: jay.krell at cornell.edu
> CC: m3devel at elegosoft.com
> Subject: Re: [M3devel] fork/cvsup
> 
> Can you sketch the approach you've taken?
> 
> 
> On 17 Mar 2010, at 11:39, Jay K wrote:
> 
> I have something working on Solaris now.
> More details after testing on Linux and Darwin.
>  
>  - Jay
>  
> From: jay.krell at cornell.edu
> To: hosking at cs.purdue.edu
> Date: Wed, 17 Mar 2010 14:01:15 +0000
> CC: m3devel at elegosoft.com
> Subject: Re: [M3devel] fork/cvsup
> 
> Exec what?
> You'd have to change the code to carefully reach the same place.
>  
>  - Jay
>  
> Subject: Re: [M3devel] fork/cvsup
> From: hosking at cs.purdue.edu
> Date: Wed, 17 Mar 2010 09:28:14 -0400
> CC: m3devel at elegosoft.com
> To: jay.krell at cornell.edu
> 
> 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
> 
> 
> 
> <m3core_atfork.txt><cvsup_forkall.txt>

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