Date: Tue, 16 Sep 2008 12:50:53 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: Andrey Chernov <ache@nagual.pp.ru> Cc: Attilio Rao <attilio@freebsd.org>, current@freebsd.org Subject: Re: Is fork() hook ever possible? Message-ID: <Pine.GSO.4.64.0809161223260.8954@sea.ntplx.net> In-Reply-To: <20080916160535.GA40676@nagual.pp.ru> References: <20080916140319.GA34447@nagual.pp.ru> <BBB443F5-042C-444E-A2F4-592B66FF2003@gid.co.uk> <20080916144502.GA39765@nagual.pp.ru> <3bbf2fe10809160753o7e5e8a78q7c6bd44c02bfd5c2@mail.gmail.com> <20080916150120.GA40087@nagual.pp.ru> <Pine.GSO.4.64.0809161125120.8677@sea.ntplx.net> <20080916160535.GA40676@nagual.pp.ru>
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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...
--
DE
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