[uml-devel] Multi-threaded applications and floating point registers

[uml-devel] Multi-threaded applications and floating point registers

[Ingo van Lil]

Hello there,

I'm experiencing some weird problems with a multi-threaded application 
I'm working on: A function which uses nothing but a single input value, 
a few constants and a couple of floating point operations returns wrong 
results from time to time. The application uses multiple threads, and 
we're suspecting that the FPU state isn't properly saved and restored 
when switching between threads within a process.

I've written a trivial test program which can be used to reproduce the 
issue (see below). In theory the program should run forever, but in UML 
it aborts after just a few seconds.

I have observed the issue both with kernel 2.6.28.8 and Busybox/uclibc 
and with the demo kernel 2.6.24-rc7 and the Fedora10 image from the UML 
home page. The problem does not happen with kernel 2.6.23.1. We're 
suspecting that the problem has been introduced by the patch [1] to stop 
saving the process FP state which has been applied just before 2.6.24-rc1.

Regards,
Ingo

[1] 
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=42daba316557e597a90a730f61c762602b7f0e0c

======================================================================

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>

static double my_abs(double d)
{
     return (d >= 0) ? d : -d;
}

static void *fp_test_thread_func(void *arg)
{
     volatile double value1 = *(double *)arg;
     volatile double value2 = *(double *)arg;
     while (1) {
         if (my_abs(value1 - value2) > 0.0001) {
             fprintf(stderr, "Inconsistent values: %lf, %lf\n", value1, 
value2);
             abort();
         }
     }
     return NULL;
}

#define NUM_THREADS 2

int main(void)
{
     pthread_t threads[NUM_THREADS];

     int i;
     for (i = 0; i < NUM_THREADS; i++) {
         double startval = i;
         pthread_create(&threads[i], NULL, fp_test_thread_func, &startval);
     }

     while (1) pause();
     return 0;
}

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Re: [uml-devel] Multi-threaded applications and floating point registers

[Stanislav Meduna]

Ingo van Lil wrote:

> I'm experiencing some weird problems with a multi-threaded application 
> I'm working on: A function which uses nothing but a single input value, 
> a few constants and a couple of floating point operations returns wrong 
> results from time to time. The application uses multiple threads, and 
> we're suspecting that the FPU state isn't properly saved and restored 
> when switching between threads within a process.

I had a similar problem between processes a few months ago,
I also posted an example, unfortunately with no resolution
nor further inquiries from the developers. I suspect the skas3
patch does not get much attention now...

Try to run the guest with mode=skas0 - in my case it eliminated
the problem and the guest is running for months without a hiccup.
If your problem is the same, it will probably help as well.

> We're suspecting that the problem has been introduced by the
> patch [1] to stop saving the process FP state which has been
> applied just before 2.6.24-rc1.

In my case it was reproducible with 2.6.23.17 as well.

Regards
-- 
                                      Stano

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Re: [uml-devel] Multi-threaded applications and floating point registers

[Ingo van Lil]

Stanislav Meduna wrote:

>> I'm experiencing some weird problems with a multi-threaded application 
>> I'm working on: A function which uses nothing but a single input value, 
>> a few constants and a couple of floating point operations returns wrong 
>> results from time to time. The application uses multiple threads, and 
>> we're suspecting that the FPU state isn't properly saved and restored 
>> when switching between threads within a process.
> 
> I had a similar problem between processes a few months ago,
> I also posted an example, unfortunately with no resolution
> nor further inquiries from the developers. I suspect the skas3
> patch does not get much attention now...

Yes, I remember the thread. I don't think we're hitting the same
problem, though: I'm not using the SKAS3 patch, and I did not notice any
kind of cross-process influence. The effect is limited to threads
sharing a single process context.

Regards,
Ingo

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