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From: radford@calgary.UUCP (Radford Neal)
Newsgroups: net.unix-wizards,net.sources
Subject: Re: When does ( alarm(1) == alarm(INFINITY) ) ?
Message-ID: <312@calgary.UUCP>
Date: Wed, 31-Dec-69 18:59:59 EST
Article-I.D.: calgary.312
Posted: Wed Dec 31 18:59:59 1969
Date-Received: Tue, 19-Mar-85 08:04:56 EST
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Organization: University of Calgary, Calgary, Alberta
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Xref: watmath net.unix-wizards:12484 net.sources:2726

> >As I read the manual (I didn't read the code) pause will only return when a
> >signal is "caught".  If the semantics were that it would also return
> >(immediately) if there were no signal handlers active (since they revert
> >when caught), the race condition would also disappear.  
> 
> This sounds nice at first, but gives rise to some even weirder
> situations... [e.g. multiple interrupt handlers]

I think the thing to do if you want to go this route is to have a 
flag in the process table entry which is set to one whenever a signal routine
is activated and cleared to zero by pause(). If pause is called when
it is one, it returns immediately (after clearing the flag).

This works as long as people have not been doing things like:

     alarm(1);
     pause();

but have instead been doing:

     alarm_caught = 0;
     alarm(1);
     while (!alarm_caught) pause(); /* alarm_caught set in signal routine */

They should have been doing this anyway, or their code will break if
there are other signal handlers active.

BTW, the routine I posted for avoiding the race (on a VAX) has a slight bug. 
Here it is again, with bug noted:

----------------------------------------------------------------------

/* PAUSE_FUDGE - Fudge routines to fix up pause race problem */

/* This module allows someone to wait until some condition has been
   made true by an interrupt routine without wasting cp time in a 
   polling loop. 

   The Unix 'pause' system call will suspend a process until an
   interrupt (i.e. signal) is received. This is not directly 
   usable in a wait loop however, since between a check for a 
   condition and a call of pause an interrupt may occur which
   would have made the condition true. So the following wait
   loop may hang up:

           while (!condition) pause();

   The following wait loop is to be used instead:

           for (;;)
           { jk_set_up_pause();
             if (condition) break;
             jk_maybe_do_pause();
           }

   The way this works is that jk_set_up_pause creates a routine
   which will perform a pause system call. jk_maybe_do_pause will
   execute this routine. The interrupt routine should be written
   to call the routine jk_disable_pause, which changes the routine
   created by jk_set_up_pause to do nothing instead of a pause.
   This is done by a change of a single Vax machine instruction to
   nop's. 

*/

static char pause_routine[5];		/* Pause system call or nop's */

/* Set up a routine to do a "pause" system call. */

jk_set_up_pause()
{ register char *p;
  p = &pause_routine[2];
  *p++ = 0274; *p++ = 035;	/* chmk $pause */   /* BUG BUG BUG BUG BUG */
  *p++ = 04;			/* ret */
}

/* Execute routine to do pause system call, unless it has been nop'ed out. */

jk_maybe_do_pause()
{ (*(void (*)())pause_routine)();
}

/* Disable pause call by replacing system call with nop's */

jk_disable_pause()
{ register char *p;
  p = &pause_routine[2];
  *p++ = 01; *p++ = 01;		/* nop; nop */
}

----------------------------------------------------------------------

The problem is that the signal may arrive between the *p++ = 0274; and the
*p++ = 035; which would lead to the final routine called by maybe_do_pause
having *half* a chmk instruction (anything might happen then, I haven't
looked to see what instruction has op-code 035). This can be fixed by
storing BOTH bytes of the chmk $pause instruction with a single assignment,
though that depends on the compiler really generating a 16-bit move rather
than two 8-bit moves (it almost certainly will).

I think the moral of all this is:

     - Don't use signal handlers if you can avoid it, especially for anything
       other than printing an error message and exiting.

     - If you have to use signal handlers, you better think REALLY carefully
       about what could happen, or you may write a program which fails 
       inexplicably once every few weeks.


     Radford Neal
     The University of Calgary