Re: Threads

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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On Oct 24 12:05, Jon TURNEY wrote:
> On 23/10/2014 16:37, Corinna Vinschen wrote:
> >On Oct 23 08:04, Ken Brown wrote:
> >>Yes, flags register corruption is exactly what Eli suggested in the other
> >>bug report I cited.
> >
> >The aforementioned patch was supposed to fix this problem and it is
> >definitely in the current 1.7.32 release...
> 
> I didn't mean to suggest otherwise, just that perhaps a similar problem
> exists now.
> 
> So I made the attached test case to explore that.  Maybe I've made an
> obvious mistake with it, but on the face of it, it seems to demonstrate
> something...
> 
> jon@tambora /
> $ gcc signal-stress.c  -Wall -O0 -g
> 
> jon@tambora /
> $ ./a
> failed: 2144210386 isn't equal to 2144210386, apparently

So it checks i and j for equality, fails, and then comes up with
"42 isn't equal to 42"?  This is weird...

> Note there is some odd load dependency. For me, it works fine when it's the
> only thing running, but when I start up something CPU intensive, it often
> fails...

That's... interesting.  I wonder if that only occurs in multi-core or
multi-CPU environments.  The fact that i and j are not the same when
testing, but then are the same when printf is called looks like a
out-of-order execution problem.

Is it possible that we have to add CPU memory barriers to the sigdelayed
function to avoid stuff like this?


Corinna


> #include <assert.h>
> #include <sys/time.h>
> #include <signal.h>
> #include <stdio.h>
> 
> long SmartScheduleInterval = 1; /* ms */
> long SmartScheduleTime = 0;
> 
> static void
> SmartScheduleTimer(int sig)
> {
>     if (sig != 0)
>        SmartScheduleTime += SmartScheduleInterval;
> }
> 
> void
> SmartScheduleStartTimer(void)
> {
>     struct itimerval timer;
>     timer.it_interval.tv_sec = 0;
>     timer.it_interval.tv_usec = SmartScheduleInterval * 1000;
>     timer.it_value.tv_sec = 0;
>     timer.it_value.tv_usec = SmartScheduleInterval * 1000;
>     setitimer(ITIMER_REAL, &timer, 0);
> }
> 
> int main()
> {
>     /* Set up the timer signal function */
>     struct sigaction act;
>     act.sa_handler = SmartScheduleTimer;
>     sigemptyset(&act.sa_mask);
>     sigaddset(&act.sa_mask, SIGALRM);
>     if (sigaction(SIGALRM, &act, 0) < 0) {
>         perror("sigaction failed");
> 	return -1;
>     }
> 
>    /* start timer */
>    SmartScheduleStartTimer();
> 
>    /* Loop forever, doing tests which should always succeed, with lots of signals */
>    int x = 0;
>    int i = 0;
>    while (1) {
>      x = i;
>      int j = x;
>      if (j != i)
>        {
>           printf("failed: %d isn't equal to %d, apparently\n", i, j);
>           break;
>        }
>      i++;
>   }
>   return 0;
> }

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-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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