[POWERPC] Avoid unpaired stwcx. on some processors

The context switch code in the kernel issues a dummy stwcx. to clear the
reservation, as recommended by the architecture.  However, some processors
can have issues if this stwcx to address A occurs while the reservation
is already held to a different address B.  To avoid this problem, the dummy
stwcx. needs to be paired with a dummy lwarx to the same address.

This adds the dummy lwarx, and creates a cpu feature bit to indicate
which cpus are affected.  Tested on mpc8641_hpcn_defconfig in
arch/powerpc; build tested in arch/ppc.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/ppc/kernel/entry.S b/arch/ppc/kernel/entry.S
index b19bfef2034d..59e77eb63338 100644
--- a/arch/ppc/kernel/entry.S
+++ b/arch/ppc/kernel/entry.S
@@ -251,6 +251,9 @@ syscall_exit_cont:
         bne-    2f
 1:
 #endif /* CONFIG_44x */
+BEGIN_FTR_SECTION
+        lwarx   r7,0,r1
+END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
         stwcx.  r0,0,r1                 /* to clear the reservation */
         lwz     r4,_LINK(r1)
         lwz     r5,_CCR(r1)
@@ -713,6 +716,9 @@ restore:
         mtctr   r11
 
         PPC405_ERR77(0,r1)
+BEGIN_FTR_SECTION
+        lwarx   r11,0,r1
+END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
         stwcx.  r0,0,r1                 /* to clear the reservation */
 
 #if !(defined(CONFIG_4xx) || defined(CONFIG_BOOKE))