Re: [PATCH v10 07/19] qspinlock: Use a simple write to grab the lock, if applicable

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On 05/08/2014 03:00 PM, Peter Zijlstra wrote:
On Wed, May 07, 2014 at 11:01:35AM -0400, Waiman Long wrote:
@@ -94,23 +94,29 @@ static inline struct mcs_spinlock *decode_tail(u32 tail)
   * can allow better optimization of the lock acquisition for the pending
   * bit holder.
   */
-#if _Q_PENDING_BITS == 8
-
  struct __qspinlock {
  	union {
  		atomic_t val;
-		struct {
  #ifdef __LITTLE_ENDIAN
+		u8	 locked;
+		struct {
  			u16	locked_pending;
  			u16	tail;
+		};
  #else
+		struct {
  			u16	tail;
  			u16	locked_pending;
-#endif
  		};
+		struct {
+			u8	reserved[3];
+			u8	locked;
+		};
+#endif
  	};
  };

+#if _Q_PENDING_BITS == 8
That doesn't make sense, that struct __qspinlock only makes sense when
_Q_PENDING_BITS == 8.

I need to use the locked field (the 2nd strcut) in get_qlock() where I grab the lock by setting the lock byte directly. Since the endian-aware structure is already in place, I reused it and have to expose it even when _Q_PENDING_BITS isn't 8. I will document that more clearly in the code to avoid this confusion.

-Longman
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