[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS



On 04/17/2014 11:56 AM, Peter Zijlstra wrote:
On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
+struct __qspinlock {
+       union {
+               atomic_t val;
+               struct {
+#ifdef __LITTLE_ENDIAN
+                       u16     locked_pending;
+                       u16     tail;
+#else
+                       u16     tail;
+                       u16     locked_pending;
+#endif
+               };
+       };
+};
+
+/**
+ * clear_pending_set_locked - take ownership and clear the pending bit.
+ * @lock: Pointer to queue spinlock structure
+ * @val : Current value of the queue spinlock 32-bit word
+ *
+ * *,1,0 ->  *,0,1
+ */
+static __always_inline void
+clear_pending_set_locked(struct qspinlock *lock, u32 val)
+{
+       struct __qspinlock *l = (void *)lock;
+
+       ACCESS_ONCE(l->locked_pending) = 1;
You lost the __constant_le16_to_cpu(_Q_LOCKED_VAL) there. The
unconditional 1 is wrong. You also have to flip the bytes in
locked_pending.

I don't think that is wrong. The lock byte is in the least significant 8 bits and the pending byte is the next higher significant 8 bits irrespective of the endian-ness. So a value of 1 in a 16-bit context means the lock byte is set, but the pending byte is cleared. The name "locked_pending" doesn't mean that locked variable is in a lower address than pending.

-Longman

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.