[Pacemaker] How to deal with unix signals in a glib mainloop (was: [Problem] The attrd does not sometimes stop.)

[renayama19661014 at ybb.ne.jp]

Hi Lars,
Hi Andrew,

I test it now in the environment that the problem reproduces with a patch of Mr. Lars.
* The patch of msgfromIPC_ll does not apply it.
* The patch of crm_trigger_prepare applies it.

The problem does not reappear on the test of several days for the moment.

I carry out a test a little more and finally report a result.
And I intend to carry out the same test with a patch of Mr. Andrew afterwards.
 * https://github.com/beekhof/pacemaker/commit/2a6b296

Best Regards,
Hideo Yamauchi.


--- On Fri, 2012/1/20, Lars Ellenberg <lars.ellenberg at linbit.com> wrote:

> On Fri, Jan 20, 2012 at 09:21:58AM +1100, Angus Salkeld wrote:
> > On 19/01/12 22:23 +0100, Lars Ellenberg wrote:
> > >On Tue, Jan 17, 2012 at 12:13:37AM +0100, Lars Ellenberg wrote:
> > >>On Tue, Jan 17, 2012 at 09:52:35AM +1100, Andrew Beekhof wrote:
> > >>>
> > >>> Ok, done:
> > >>>
> > >>> https://github.com/beekhof/pacemaker/commit/2a6b296
> > >>>
> > >>> If I'm adding voodoo, I at least want the reason well documented so it
> > >>> can be removed again if the reason goes away.
> > >>
> > >>That about sums it up, then ;-)
> > >
> > >But as having to do this was just "too ugly to be true",
> > >I dug a little deeper...
> > >
> > >The way to do this is obviously to use the glib api ;-)
> > >http://developer.gnome.org/glib/2.30/glib-UNIX-specific-utilities-and-integration.html#g-unix-signal-add-full
> > >
> > >(Since glib 2.30, yay; if you don't have that yet, read on anyways)
> > >
> > >What it does internally, and what other people have also done for a long
> > >time  to solve this and similar problems, is:
> > >
> > >Add to the main context a "wakeup pipe",
> > >which is an eventfd if available,
> > >or an actual pipe if not.
> > >If it is a pipe, set those file descriptors non-blocking.
> > >And, of course, add the evenfd (or the read end of the pipe)
> > >to the poll loop (with default priority, btw,
> > >which is good enough to have the poll terminate).
> > >
> > >That is done internally when creating the main context.
> > >http://git.gnome.org/browse/glib/tree/glib/gmain.c#n548
> > >http://git.gnome.org/browse/glib/tree/glib/gwakeup.c#n138
> > >
> > >(the line numbers are correct for glib master as of today,
> > >which should correspond to 41fbf42)
> > >
> > >The g_unix_signal_handler then sets the triggers variables,
> > >and calls g_wakeup_signal(that internal wakeup source),
> > >which simply posts and event to the eventfd,
> > >or does a short (1 byte) write to the write end of the pipe.
> > >http://git.gnome.org/browse/glib/tree/glib/gmain.c#n4442
> > >http://git.gnome.org/browse/glib/tree/glib/gwakeup.c#n230
> > >
> > >Problem solved, without having to do a full check() everything,
> > >prepare() everything, and poll() again cycle every 500ms.
> > >
> > >"back in those days", when this mechanism was not really there,
> > >you could do all that "by hand".
> > >And people did. Very common idiom in glib and other mainloop
> > >applications, also frequently used to "signal" availability of work
> > >or completion of tasks between threads.
> > >
> > >static int my_wakeup_fds[2] = { -1, -1 };
> > >
> > >Then just pipe2(my_wakeup_fds, O_NONBLOCK), add my_wakeup_fds[0] as
> > >normal read fd source, and add a write(my_wakeup_fds[1], "", 1); to the
> > >signal handlers.
> > 
> > signalfd makes this much easier too "man 2 signalfd"
> 
> See https://bugzilla.gnome.org/show_bug.cgi?id=652072#c32
> following (or the whole bug, if you like).
> 
> Also, pipes are portable.
> 
>     Lars
> 
> 
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