Messages in this thread Patch in this message |  | | | Date | Sat, 18 Nov 2000 08:26:27 -0800 (PST) | | From | Linus Torvalds <> | | Subject | Re: Freeze on FPU exception with Athlon |
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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 { /* - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/
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