Messages in this thread | | | Date | Wed, 17 Aug 2011 22:58:23 +0200 | From | Richard Weinberger <> | Subject | Floating point problems on UML - help needed |
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Hi,
Gunnar reported that some Java program does not work proper within UserModeLinux. After looking closer at the problem I was able to reduce it to a small C program. (Program is attached.)
It looks like FPU registers get sometimes lost after switching between two or more threads. It also happens not always, that's why my test program contains a infinite loop. After a few million iterations the program abort()s.
I can reproduce the issue on both x86 and x86_64, the host's or UML's kernel version does not matter. I've tested 2.6.18 to 3.1-rc2.
Interestingly the problem occurs not on my old Pentium 4 machines. One P4 has HT the other not. Only "newer" CPUs are affected.
First I thought it's a race in _switch_to(), but adding unblock/block_signals() did not help. Currently I'm running out of ideas. I'm not an expert in this area of UML. :-(
Any idea what goes wrong here?
Thanks, //richard #include <stdio.h> #include <stdlib.h> #include <pthread.h>
static int xxx(float f) { if(f <= 0.0){ printf("wrong f!: %f\n", f); return -1; }
return 0; }
static void *fun(void *arg) { float f = 5.0;
for(;;){ if(xxx(f) < 0) abort(); }
return NULL; }
int main() { pthread_t t1, t2;
pthread_create(&t1, NULL, fun, NULL); pthread_create(&t2, NULL, fun, NULL);
pthread_join(t1, NULL); pthread_join(t2, NULL);
return 0; }
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