[PATCH v2 3/3] armv6: fix armv6 build issue

[Alex Shi]

Arm v6 can not simulate cmpxchg1 func, so we have to use cmpxchg4 on it.

   arm-linux-gnueabi-ld: mm/page_alloc.o: in function `set_pfnblock_flags_mask':
   (.text+0x32b4): undefined reference to `__bad_cmpxchg'
   arm-linux-gnueabi-ld: (.text+0x32e0): undefined reference to `__bad_cmpxchg'
   arm-linux-gnueabi-ld: drivers/net/ethernet/aquantia/atlantic/hw_atl/hw_atl_b0.o: in function `hw_atl_b0_get_mac_temp':
   hw_atl_b0.c:(.text+0x30fc): undefined reference to `__bad_udelay'

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
---
 mm/page_alloc.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7da09d66233b..c09146a8946c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -517,7 +517,11 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 {
 	unsigned char *bitmap;
 	unsigned long bitidx, byte_bitidx;
+#ifdef	CONFIG_CPU_V6
+	unsigned long old_byte, byte;
+#else
 	unsigned char old_byte, byte;
+#endif
 
 	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != BITS_PER_BYTE);
 	BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
@@ -532,9 +536,18 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 	mask <<= bitidx;
 	flags <<= bitidx;
 
+#ifdef	CONFIG_CPU_V6
+	byte = (unsigned long)READ_ONCE(bitmap[byte_bitidx]);
+#else
 	byte = READ_ONCE(bitmap[byte_bitidx]);
+#endif
 	for (;;) {
+#ifdef	CONFIG_CPU_V6
+		/* arm v6 has no cmpxchgb function, so still false sharing long word */
+		old_byte = cmpxchg((unsigned long*)&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
+#else
 		old_byte = cmpxchg(&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
+#endif
 		if (byte == old_byte)
 			break;
 		byte = old_byte;
-- 
1.8.3.1