[PARISC] futex: special case cmpxchg NULL in kernel space

Commit a0c1e907 added code to futex.c
to detect whether futex_atomic_cmpxchg_inatomic was implemented at run
time:

+       curval = cmpxchg_futex_value_locked(NULL, 0, 0);
+       if (curval == -EFAULT)
+               futex_cmpxchg_enabled = 1;

This is bogus on parisc, since page zero in kernel virtual space is the
gateway page for syscall entry, and should not be read from the kernel.
(That, and we really don't like the kernel faulting on its own address
 space...)
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>

include/asm-parisc/futex.h

@@ -56,6 +56,12 @@ futex_atomic_cmpxchg_inatomic(int __user *uaddr, int oldval, int newval)
 	int err = 0;
 	int uval;
 
+	/* futex.c wants to do a cmpxchg_inatomic on kernel NULL, which is
+	 * our gateway page, and causes no end of trouble...
+	 */
+	if (segment_eq(KERNEL_DS, get_fs()) && !uaddr)
+		return -EFAULT;
+
 	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
 		return -EFAULT;
 
@@ -67,5 +73,5 @@ futex_atomic_cmpxchg_inatomic(int __user *uaddr, int oldval, int newval)
 	return uval;
 }
 
-#endif
-#endif
+#endif /*__KERNEL__*/
+#endif /*_ASM_PARISC_FUTEX_H*/