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Re: [Xen-devel] [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS
- To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- From: Waiman Long <waiman.long@xxxxxx>
- Date: Thu, 17 Apr 2014 17:46:27 -0400
- Cc: linux-arch@xxxxxxxxxxxxxxx, Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>, Gleb Natapov <gleb@xxxxxxxxxx>, kvm@xxxxxxxxxxxxxxx, Scott J Norton <scott.norton@xxxxxx>, x86@xxxxxxxxxx, Paolo Bonzini <paolo.bonzini@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, Ingo Molnar <mingo@xxxxxxxxxx>, Chegu Vinod <chegu_vinod@xxxxxx>, David Vrabel <david.vrabel@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Oleg Nesterov <oleg@xxxxxxxxxx>
- Delivery-date: Thu, 17 Apr 2014 21:47:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
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
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