The slot lock is a bit operation on the whole __lock word, which
flags and ac_time alias as two u32s. On little-endian the lock bit
lands in the position ZRAM_ENTRY_LOCK reserves in flags, so the
aliasing works out. On 64-bit big-endian it lands in ac_time
instead: with ZRAM_TRACK_ENTRY_ACTIME enabled, storing the access
time from mark_slot_accessed() or slot_free() wipes out the held
lock bit, letting another CPU take the same slot lock; an access
time value with that bit set makes the slot look locked forever.

Shift the lock bit into the flags half of the word on big-endian
64-bit.

Fixes: 2e8ff2f51dde ("zram: use u32 for entry ac_time tracking")
Cc: stable@vger.kernel.org
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 drivers/block/zram/zram_drv.c |  6 +++---
 drivers/block/zram/zram_drv.h | 14 ++++++++++++++
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index d09fdca49cbd..a9b3bb1d3bef 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -74,7 +74,7 @@ static __must_check bool slot_trylock(struct zram *zram, unsigned long index)
 {
 	unsigned long *lock = &zram->table[index].__lock;
 
-	if (!test_and_set_bit_lock(ZRAM_ENTRY_LOCK, lock)) {
+	if (!test_and_set_bit_lock(ZRAM_ENTRY_LOCK_BIT, lock)) {
 		mutex_acquire(&zram->table_lock_map, 0, 1, _RET_IP_);
 		lock_acquired(&zram->table_lock_map, _RET_IP_);
 		return true;
@@ -88,7 +88,7 @@ static void slot_lock(struct zram *zram, unsigned long index)
 	unsigned long *lock = &zram->table[index].__lock;
 
 	mutex_acquire(&zram->table_lock_map, 0, 0, _RET_IP_);
-	wait_on_bit_lock(lock, ZRAM_ENTRY_LOCK, TASK_UNINTERRUPTIBLE);
+	wait_on_bit_lock(lock, ZRAM_ENTRY_LOCK_BIT, TASK_UNINTERRUPTIBLE);
 	lock_acquired(&zram->table_lock_map, _RET_IP_);
 }
 
@@ -97,7 +97,7 @@ static void slot_unlock(struct zram *zram, unsigned long index)
 	unsigned long *lock = &zram->table[index].__lock;
 
 	mutex_release(&zram->table_lock_map, _RET_IP_);
-	clear_and_wake_up_bit(ZRAM_ENTRY_LOCK, lock);
+	clear_and_wake_up_bit(ZRAM_ENTRY_LOCK_BIT, lock);
 }
 
 static inline bool init_done(struct zram *zram)
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 4fddc582f3b8..7a55d751417e 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -15,6 +15,7 @@
 #ifndef _ZRAM_DRV_H_
 #define _ZRAM_DRV_H_
 
+#include <asm/byteorder.h>
 #include <linux/rwsem.h>
 #include <linux/zsmalloc.h>
 
@@ -57,6 +58,19 @@ enum zram_pageflags {
 	__NR_ZRAM_PAGEFLAGS,
 };
 
+/*
+ * The slot lock is a bit-wait lock on the whole __lock word, while
+ * flags and ac_time alias that word as two u32s.  The lock bit must
+ * land in the slot that ZRAM_ENTRY_LOCK reserves in attr.flags; on
+ * 64-bit big-endian the flags word maps to the upper half of __lock,
+ * so the bit position has to be shifted up.
+ */
+#if defined(CONFIG_64BIT) && defined(__BIG_ENDIAN)
+#define ZRAM_ENTRY_LOCK_BIT  (ZRAM_ENTRY_LOCK + 32)
+#else
+#define ZRAM_ENTRY_LOCK_BIT  ZRAM_ENTRY_LOCK
+#endif
+
 /*
  * Allocated for each disk page.  We use bit-lock (ZRAM_ENTRY_LOCK bit
  * of flags) to save memory.  There can be plenty of entries and standard
-- 
2.55.0

On (26/08/10 21:22), David Carlier wrote:
> The slot lock is a bit operation on the whole __lock word, which
> flags and ac_time alias as two u32s. On little-endian the lock bit
> lands in the position ZRAM_ENTRY_LOCK reserves in flags, so the
> aliasing works out. On 64-bit big-endian it lands in ac_time
> instead: with ZRAM_TRACK_ENTRY_ACTIME enabled, storing the access
> time from mark_slot_accessed() or slot_free() wipes out the held
> lock bit, letting another CPU take the same slot lock; an access
> time value with that bit set makes the slot look locked forever.
> 
> Shift the lock bit into the flags half of the word on big-endian
> 64-bit.
> 
> Fixes: 2e8ff2f51dde ("zram: use u32 for entry ac_time tracking")
> Cc: stable@vger.kernel.org
> Signed-off-by: David Carlier <devnexen@gmail.com>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>