[PATCH 3.16 127/204] parisc: Fix double-word compare and exchange in LWS code on 32-bit kernels

[Ben Hutchings]

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: John David Anglin <dave.anglin@xxxxxxxx>

commit 374b3bf8e8b519f61eb9775888074c6e46b3bf0c upstream.

As discussed on the debian-hppa list, double-wordcompare and exchange
operations fail on 32-bit kernels.  Looking at the code, I realized that
the ",ma" completer does the wrong thing in the  "ldw,ma  4(%r26), %r29"
instruction.  This increments %r26 and causes the following store to
write to the wrong location.

Note by Helge Deller:
The patch applies cleanly to stable kernel series if this upstream
commit is merged in advance:
f4125cfdb300 ("parisc: Avoid trashing sr2 and sr3 in LWS code").

Signed-off-by: John David Anglin <dave.anglin@xxxxxxxx>
Tested-by: Christoph Biedl <debian.axhn@xxxxxxxxxxxxxxxxxx>
Fixes: 89206491201c ("parisc: Implement new LWS CAS supporting 64 bit operations.")
Signed-off-by: Helge Deller <deller@xxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/parisc/kernel/syscall.S | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/arch/parisc/kernel/syscall.S
+++ b/arch/parisc/kernel/syscall.S
@@ -742,7 +742,7 @@ lws_compare_and_swap_2:
 10:	ldd	0(%sr3,%r25), %r25
 11:	ldd	0(%sr3,%r24), %r24
 #else
-	/* Load new value into r22/r23 - high/low */
+	/* Load old value into r22/r23 - high/low */
 10:	ldw	0(%sr3,%r25), %r22
 11:	ldw	4(%sr3,%r25), %r23
 	/* Load new value into fr4 for atomic store later */
@@ -834,11 +834,11 @@ cas2_action:
 	copy	%r0, %r28
 #else
 	/* Compare first word */
-19:	ldw,ma	0(%sr3,%r26), %r29
+19:	ldw	0(%sr3,%r26), %r29
 	sub,=	%r29, %r22, %r0
 	b,n	cas2_end
 	/* Compare second word */
-20:	ldw,ma	4(%sr3,%r26), %r29
+20:	ldw	4(%sr3,%r26), %r29
 	sub,=	%r29, %r23, %r0
 	b,n	cas2_end
 	/* Perform the store */