Re: busyloop in sigchld_handler

[storm@cua.dk (Kim F. Storm)]

Sam Steingold <sds@gnu.org> writes:

> The following message is a courtesy copy of an article
> that has been posted to gmane.emacs.devel as well.
>
>> * Kim F. Storm <fgbez@phn.qx> [2007-03-11 20:39:25 +0100]:
>>
>> Sam Steingold <sds@gnu.org> writes:
>>
>> Absolutely brilliant!  I can confirm that it works.
>
> Thanks!

Actually, I'm not quite sure your patch works.

It seems the unresponsiveness is just delayed some seconds -- at
least sometimes.

I don't know what the system is doing -- flushing the buffer
cache or some such?

GNU Emacs 22.0.95.13 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
Linux kernel: 2.4.20-8


>
>> But can you explain why it works?
>
> wait3 is a system call, which, when invoked in a loop, prevents the
> kernel from doing anything else (in this case, sending SIGCHLD to
> emacs). sleep allows the kernel some time to pass the signal.
>

I tried to verify that there is a loop, but I can't.

Apply the patch below, start emacs -Q, run M-x compile, and look at
sigchld_pid array.  I get this:

(gdb) p sigchld_count
$1 = 2
(gdb) p sigchld_sleep
$2 = 8
(gdb) p sigchld_pid
$3 = {1173651414, 0, 5453, 0, 1173651414, 0, -1, 10, 0 <repeats 12 times>}

So I'm puzzled what's going on.


*** process.c	11 Mar 2007 20:32:23 +0100	1.501
--- process.c	11 Mar 2007 23:24:08 +0100	
***************
*** 6468,6473 ****
--- 6468,6478 ----
     indirectly; if it does, that is a bug  */
  
  #ifdef SIGCHLD
+ 
+ long sigchld_pid[20];
+ int sigchld_count = 0;
+ int sigchld_sleep = 0;
+ 
  SIGTYPE
  sigchld_handler (signo)
       int signo;
***************
*** 6495,6505 ****
  #define WUNTRACED 0
  #endif /* no WUNTRACED */
        /* Keep trying to get a status until we get a definitive result.  */
        do
  	{
!           sleep (1);
  	  errno = 0;
  	  pid = wait3 (&w, WNOHANG | WUNTRACED, 0);
  	}
        while (pid < 0 && errno == EINTR);
  
--- 6500,6522 ----
  #define WUNTRACED 0
  #endif /* no WUNTRACED */
        /* Keep trying to get a status until we get a definitive result.  */
+ 
+       sigchld_count++;
        do
  	{
! 	  unsigned long t1, t2;
!           /* sleep (1); */
! 	  time(&t1);
  	  errno = 0;
  	  pid = wait3 (&w, WNOHANG | WUNTRACED, 0);
+ 	  if (sigchld_sleep < 15)
+ 	    {
+ 	      time(&t2);
+ 	      sigchld_pid[sigchld_sleep++] = t1;
+ 	      sigchld_pid[sigchld_sleep++] = t2 - t1;
+ 	      sigchld_pid[sigchld_sleep++] = pid;
+ 	      sigchld_pid[sigchld_sleep++] = errno;
+ 	    }
  	}
        while (pid < 0 && errno == EINTR);
  


--
Kim F. Storm <storm@cua.dk> http://www.cua.dk