1 files changed, 25 insertions, 5 deletions

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index f956ede7f90d..6c5da483966b 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -1,5 +1,5 @@
 /*
- * Queue read/write lock
+ * Queued read/write locks
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -22,6 +22,26 @@
 #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
@@ -107,10 +127,10 @@ void queue_write_lock_slowpath(struct qrwlock *lock)
          * or wait for a previous writer to go away.
          */
         for (;;) {
-                cnts = atomic_read(&lock->cnts);
-                if (!(cnts & _QW_WMASK) &&
-                    (atomic_cmpxchg(&lock->cnts, cnts,
-                                    cnts | _QW_WAITING) == cnts))
+                struct __qrwlock *l = (struct __qrwlock *)lock;
+
+                if (!READ_ONCE(l->wmode) &&
+                   (cmpxchg(&l->wmode, 0, _QW_WAITING) == 0))
                         break;
 
                 cpu_relax_lowlatency();