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SubjectRe: Freeze on FPU exception with Athlon


On Sat, 18 Nov 2000, Brian Gerst wrote:
>
> I get Floating Point Exception (core dumped), but I needed to use the
> modified program below to keep GCC from optimizing the division away as
> a constant. This is on test11-pre5.

I'm starting to suspect that it's really a combination of three things:
- 3dnow optimization (ie you have to compile the kernel with Athlon
support)
- pending, but not yet noticed, FPU exceptions.
- a bug/feature in the kernel, where a process exit does not bother to
clear the FPU, only marks it as "unused".

If I'm right, the proper test-program should be something like

int main(int argc, char **argv)
{
asm("fldcw %0": :"m" (0));
asm("fldz ; fld1 ; fdiv");
sleep(1);
return 0;
}

where it's important that we do not wait for the result of the fdiv, we
just exit after having caused a pending exception (and you cannot do this
reliably from C code - depending on compiler version and optimizations
gcc may try to write the bad value back to memory etc).

Now, with the pending exception, do a 3dnow MMX memcpy() - which will
clear the TS bit (because it decides that the FP state can be thrown
away and doesn't need to do a full save/restore) and start using the FPU.
Boom. Instant FP exception. With the exception handler deciding that
nobody owns the FP state, and thus doing nothing sane.

If I'm right (and I'm _always_ right), the following patch would make a
difference.

Markus?

Linus

----
--- v2.4.0-test10/linux/arch/i386/kernel/traps.c Tue Oct 31 12:42:26 2000
+++ linux/arch/i386/kernel/traps.c Fri Nov 17 21:52:55 2000
@@ -643,6 +640,12 @@
asmlinkage void do_coprocessor_error(struct pt_regs * regs, long error_code)
{
ignore_irq13 = 1;
+
+ /* Due to lazy error handling, we might have false pending errors! */
+ if (!current->used_math) {
+ init_fpu();
+ return;
+ }
math_error((void *)regs->eip);
}

@@ -700,6 +703,12 @@
if (cpu_has_xmm) {
/* Handle SIMD FPU exceptions on PIII+ processors. */
ignore_irq13 = 1;
+
+ /* Due to lazy error handling, we might have false pending errors! */
+ if (!current->used_math) {
+ init_fpu();
+ return;
+ }
simd_math_error((void *)regs->eip);
} else {
/*
-
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