Found by Bodo Stroesser. Description from Bodo:

>>
On i386, if a signal handler is started, the kernel saves the fpu-state
of the interrupted routine in the sigcontext on the stack. Calling
unlazy_fpu() and setting current->used_math=0, the kernel supplies the
signal-handler with a cleared virtual fpu.
On sigreturn(), the old fpu-state of the interrupted routine is
restored.

If a process never used the fpu, it virtually has a cleared fpu.
If such a process is interrupted by a signal handler, no fpu-context is
saved and sigcontext->fpstate is set to NULL.

Assume, that the signal handler uses the fpu. Then, AFAICS, on sigreturn
current->used_math will be 1. Since sigcontext->fpstate still is NULL,
restore_sigcontext() doesn't call restore_i387(). Thus, no
clear_fpu() is done, current->used_math is not reset.

Now, the interrupted processes fpu no longer is cleared!
<<

Fix by AK. Just clear the FPU again when this happens.

patch for i386 and x86-64.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>