Re: timer + fpu stuff locks up computer

[Sergey Vlasov <vsu@altlinux.ru>]

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On Sat, Jun 12, 2004 at 07:14:22PM +0400, Sergey Vlasov wrote:
> If the FPU state belong to the userspace process, kernel_fpu_begin()
> is safe even if some exceptions are pending.  However, after
> __clear_fpu() the FPU is "orphaned", and kernel_fpu_begin() does
> nothing with it.
> 
> Replacing fwait with fnclex instead of removing it completely should
> avoid the fault later.

Yes, it seems to be enough.  Another case where it looks like FPU
might be "orphaned" is exit(); however, it is handled as a normal task
switch, __switch_to() calls __unlazy_fpu(), which clears pending
exceptions.

I'm still not sure what to do about possibly lost FP exceptions.  This
can happen in two cases:

1) Program calls execve() while an FP exception is pending.

   In this case clear_fpu() is called when the original executable is
   already destroyed.  Even if we generate a SIGFPE in this case, it
   would be delivered to the new executable.

2) Program returns from a signal handler while an FP exception is
   pending.

   In this case at clear_fpu() time restore_sigcontext() has already
   wiped out all state of the signal handler, so the SIGFPE would
   appear to be raised from the program code at the point where it was
   interrupted by the handled signal.

Signed-Off-By: Sergey Vlasov <vsu@altlinux.ru>

--- linux-2.6.6/include/asm-i386/i387.h.fp-lockup	2004-05-10 06:33:06 +0400
+++ linux-2.6.6/include/asm-i386/i387.h	2004-06-12 22:02:58 +0400
@@ -48,10 +48,17 @@
 		save_init_fpu( tsk ); \
 } while (0)
 
+/*
+ * There might be some pending exceptions in the FP state at this point.
+ * However, it is too late to report them: this code is called during execve()
+ * (when the original executable is already gone) and during sigreturn() (when
+ * the signal handler context is already lost).  So just clear them to prevent
+ * problems later.
+ */
 #define __clear_fpu( tsk )					\
 do {								\
 	if ((tsk)->thread_info->status & TS_USEDFPU) {		\
-		asm volatile("fwait");				\
+		asm volatile("fnclex");				\
 		(tsk)->thread_info->status &= ~TS_USEDFPU;	\
 		stts();						\
 	}							\


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