Is fork() hook ever possible?

[Kostik Belousov]

On Tue, Sep 16, 2008 at 12:50:53PM -0400, Daniel Eischen wrote:
> 
> [ Trimmed ]
> 
> On Tue, 16 Sep 2008, Andrey Chernov wrote:
> 
> >On Tue, Sep 16, 2008 at 11:36:03AM -0400, Daniel Eischen wrote:
> >
> >>Well, you could speed up getpid() by having libc wrap all fork()
> >>variants.  The idea is that getpid() would only call __sys_getpid()
> >>the first time it was called and then only after a fork().  It
> >>would return the saved process id for all other cases.
> >
> >Yes, speeding up getpid() by caching its pid is nice idea.
> >But I am completely unaware how to create syscall wrappers inside libc. :(
> >I think about something like that:
> >
> >__weak_reference(_fork, fork);
> 
> I think you'll have to implement it as __fork() in libc, with
> _fork and fork both being weak references to __fork() in libc.  The
> thread libraries will have to call __fork() instead of __sys_fork()
> by implementing "fork" as _fork() and providing a weak reference
> from fork to _fork.  You can see wait() as an example.
> 
> Probably rfork() and vfork() will need to be handled as well,
> though I don't think that the thread libraries care about these.
> 
> >But how it will coexists with the same __weak in thread/thr_fork.c ?
> >Are some threading locks required in this code?
> 
> I think you can do it without locks.  After a fork() you are
> single threaded so you can easily set/clear __cur_thread.
> Otherwise, the worst case is that multiple threads will call
> _sys_getpid() simultaneously the first time, but as long as
> you atomically update __cur_thread, it won't matter - each
> thread will have retrieved the same exact process id so it
> is okay if they all update __cur_thread.
> 
> 	pid_t
> 	__getpid(void)
> 	{
> 
> 		if (__cur_thread != -1)
> 			return (__cur_thread);
> 
> 		atomic_set_32(&__cur_thread, __sys_getpid());
> 			return (__cur_thread);
> 	}
> 	__weak_reference(__getpid, getpid);
> 	__weak_reference(__getpid, _getpid);
> 
> Or something like that...

Do not forget about rfork(). Not sure about rfork_thread().
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