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Subject[PATCH v3] locking/qrwlock: Let qrwlock has same layout regardless of the endian
Date
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>
---
change from v2:
change macro's name, add comments.
change from v1:
A typo fix which is really bad...
thanks Will for the carefull review. :)
---
include/asm-generic/qrwlock.h | 15 +++++++++++----
kernel/locking/qrwlock.c | 11 +++++------
2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 54a8e65..28fb94a 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -27,11 +27,18 @@
/*
* Writer states & reader shift and bias
*/
-#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 (1U << _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 (1U << _QR_SHIFT)

/*
* External function declarations
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index fec0823..0502e3a 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -30,18 +30,16 @@ 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;
};

+/* get the value of wmode when cnts is v */
+#define _QW_BYTEVAL(v) ((v) >> _QW_SHIFT)
+
/**
* rspin_until_writer_unlock - inc reader count & spin until writer is gone
* @lock : Pointer to queue rwlock structure
@@ -127,7 +125,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
struct __qrwlock *l = (struct __qrwlock *)lock;

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

cpu_relax_lowlatency();
--
1.9.1
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