[ARM] 3309/1: disable the pre-ARMv5 NPTL kernel helper in the non MMU case

Patch from Nicolas Pitre

The cmpxchg emulation on pre-ARMv5 relies on user code executed from a
kernel address.  If the operation cannot complete atomically, it is
aborted from the usr_entry macro by clearing the Z flag.  This clearing
of the Z flag is done whenever the user pc is above TASK_SIZE.

However this "pc >= TASK_SIZE" test cannot work in the non MMU case.
Worse: the current code will corrupt the Z flag on every entry to the
kernel.

Let's disable it in the non MMU case for now.  Using NPTL on non MMU
targets needs to be worked out anyway.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index d401d908c46..f248bbfe745 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -333,10 +333,14 @@ __pabt_svc:
                                         @ from the exception stack
 
 #if __LINUX_ARM_ARCH__ < 6 && !defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
+#ifndef CONFIG_MMU
+#warning "NPTL on non MMU needs fixing"
+#else
         @ make sure our user space atomic helper is aborted
         cmp     r2, #TASK_SIZE
         bichs   r3, r3, #PSR_Z_BIT
 #endif
+#endif
 
         @
         @ We are now ready to fill in the remaining blanks on the stack:
@@ -756,12 +760,18 @@ __kuser_cmpxchg:				@ 0xffff0fc0
          * exception happening just after the str instruction which would
          * clear the Z flag although the exchange was done.
          */
+#ifdef CONFIG_MMU
         teq     ip, ip                  @ set Z flag
         ldr     ip, [r2]                @ load current val
         add     r3, r2, #1              @ prepare store ptr
         teqeq   ip, r0                  @ compare with oldval if still allowed
         streq   r1, [r3, #-1]!          @ store newval if still allowed
         subs    r0, r2, r3              @ if r2 == r3 the str occured
+#else
+#warning "NPTL on non MMU needs fixing"
+        mov     r0, #-1
+        adds    r0, r0, #0
+#endif
         mov     pc, lr
 
 #else