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SubjectRe: [PATCH v3] locking/qrwlock: Let qrwlock has same layout regardless of the endian
On Mon, Jun 20, 2016 at 02:20:52PM +0800, Pan Xinhui wrote:
> This patch aims to get rid of endianness in queued_write_unlock(). We
> want to set __qrwlock->wmode to NULL, however the address is not
> &lock->cnts in big endian machine. That causes queued_write_unlock()
> write NULL to the wrong field of __qrwlock.
>
> Actually qrwlock can have same layout, IOW we can remove the #if
> __little_endian in struct __qrwlock. With such modification, we only
> need define some _QW* and _QR* with corresponding values in different
> endian systems.
>
> Suggested-by: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
> Acked-by: Waiman Long <Waiman.Long@hpe.com>
> ---

Urgh, I hate this stuff :/

OK, so I poked at this a bit and I ended up with the below; but now
qrwlock and qspinlock are inconsistent; although I suspect qspinlock is
similarly busted wrt endian muck.

Not sure what to do..

--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -25,13 +25,31 @@
#include <asm-generic/qrwlock_types.h>

/*
- * Writer states & reader shift and bias
+ * Writer states & reader shift and bias.
+ *
+ * | +0 | +1 | +2 | +3 |
+ * ----+----+----+----+----+
+ * LE | 12 | 34 | 56 | 78 | 0x12345678
+ * ----+----+----+----+----+
+ * BE | 78 | 56 | 34 | 12 | 0x12345678
+ * ----+----+----+----+----+
+ * | wr | rd |
+ * +----+----+----+----+
+ *
*/
-#define _QW_WAITING 1 /* A writer is waiting */
-#define _QW_LOCKED 0xff /* A writer holds the lock */
-#define _QW_WMASK 0xff /* Writer mask */
+#ifdef __LITTLE_ENDIAN
#define _QR_SHIFT 8 /* Reader count shift */
-#define _QR_BIAS (1U << _QR_SHIFT)
+#define _QW_SHIFT 0 /* Writer mode shift */
+#else
+#define _QR_SHIFT 0 /* Reader count shift */
+#define _QW_SHIFT 24 /* Writer mode shift */
+#endif
+
+#define _QW_WAITING (0x01U << _QW_SHIFT) /* A writer is waiting */
+#define _QW_LOCKED (0xffU << _QW_SHIFT) /* A writer holds the lock */
+#define _QW_WMASK (0xffU << _QW_SHIFT) /* Writer mask */
+
+#define _QR_BIAS (0x01U << _QR_SHIFT)

/*
* External function declarations
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -22,26 +22,6 @@
#include <linux/hardirq.h>
#include <asm/qrwlock.h>

-/*
- * This internal data structure is used for optimizing access to some of
- * the subfields within the atomic_t cnts.
- */
-struct __qrwlock {
- union {
- atomic_t cnts;
- struct {
-#ifdef __LITTLE_ENDIAN
- u8 wmode; /* Writer mode */
- u8 rcnts[3]; /* Reader counts */
-#else
- u8 rcnts[3]; /* Reader counts */
- u8 wmode; /* Writer mode */
-#endif
- };
- };
- arch_spinlock_t lock;
-};
-
/**
* rspin_until_writer_unlock - inc reader count & spin until writer is gone
* @lock : Pointer to queue rwlock structure
@@ -124,10 +104,10 @@ void queued_write_lock_slowpath(struct q
* or wait for a previous writer to go away.
*/
for (;;) {
- struct __qrwlock *l = (struct __qrwlock *)lock;
+ u8 *wr = (u8 *)lock;

- if (!READ_ONCE(l->wmode) &&
- (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
+ if (!READ_ONCE(*wr) &&
+ (cmpxchg_relaxed(wr, 0, _QW_WAITING >> _QW_SHIFT) == 0))
break;

cpu_relax_lowlatency();
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