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Subject[tip:locking/core] locking/qrwlock: Fix write unlock bug on big endian systems
Commit-ID:  2db34e8bf9a22f4e38b29deccee57457bc0e7d74
Gitweb: http://git.kernel.org/tip/2db34e8bf9a22f4e38b29deccee57457bc0e7d74
Author: pan xinhui <xinhui.pan@linux.vnet.ibm.com>
AuthorDate: Mon, 18 Jul 2016 17:47:39 +0800
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 10 Aug 2016 14:13:27 +0200

locking/qrwlock: Fix write unlock bug on big endian systems

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.

So implement __qrwlock_write_byte() which returns the correct
__qrwlock->wmode address.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman.Long@hpe.com
Cc: arnd@arndb.de
Cc: boqun.feng@gmail.com
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1468835259-4486-1-git-send-email-xinhui.pan@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
include/asm-generic/qrwlock.h | 27 +++++++++++++++++++++++++--
1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 54a8e65..7d026bf 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -25,7 +25,20 @@
#include <asm-generic/qrwlock_types.h>

/*
- * Writer states & reader shift and bias
+ * Writer states & reader shift and bias.
+ *
+ * | +0 | +1 | +2 | +3 |
+ * ----+----+----+----+----+
+ * LE | 78 | 56 | 34 | 12 | 0x12345678
+ * ----+----+----+----+----+
+ * | wr | rd |
+ * +----+----+----+----+
+ *
+ * ----+----+----+----+----+
+ * BE | 12 | 34 | 56 | 78 | 0x12345678
+ * ----+----+----+----+----+
+ * | rd | wr |
+ * +----+----+----+----+
*/
#define _QW_WAITING 1 /* A writer is waiting */
#define _QW_LOCKED 0xff /* A writer holds the lock */
@@ -134,12 +147,22 @@ static inline void queued_read_unlock(struct qrwlock *lock)
}

/**
+ * __qrwlock_write_byte - retrieve the write byte address of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ * Return: the write byte address of a queue rwlock
+ */
+static inline u8 *__qrwlock_write_byte(struct qrwlock *lock)
+{
+ return (u8 *)lock + 3 * IS_BUILTIN(CONFIG_CPU_BIG_ENDIAN);
+}
+
+/**
* queued_write_unlock - release write lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
static inline void queued_write_unlock(struct qrwlock *lock)
{
- smp_store_release((u8 *)&lock->cnts, 0);
+ smp_store_release(__qrwlock_write_byte(lock), 0);
}

/*
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