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Re: [Xen-devel] [PATCH v10 07/19] qspinlock: Use a simple write to grab the lock, if applicable
- To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- From: Waiman Long <waiman.long@xxxxxx>
- Date: Fri, 09 May 2014 21:05:25 -0400
- Cc: linux-arch@xxxxxxxxxxxxxxx, Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>, Oleg Nesterov <oleg@xxxxxxxxxx>, 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>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
- Delivery-date: Sat, 10 May 2014 01:06:08 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
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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