linux-kernel - Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR

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Date:	Fri, 18 Apr 2014 10:27:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS

On Thu, Apr 17, 2014 at 05:46:27PM -0400, Waiman Long wrote:
> 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;

		char bytes[4];

> >>+		struct {
> >>+#ifdef __LITTLE_ENDIAN
> >>+			u16	locked_pending;
> >>+			u16	tail;
> >>+#else
> >>+			u16	tail;
> >>+			u16	locked_pending;
> >>+#endif
> >>+		};

		struct {
#ifdef __LITTLE_ENDIAN
			u8	locked;
#else
			u8	res[3];
			u8	locked;
#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.

val            is LE bytes[0,1,2,3] BE [3,2,1,0]
locked_pending is LE bytes[0,1]     BE     [1,0]
locked            LE bytes[0]       BE       [0]

That does mean that the LSB of BE locked_pending is bytes[1].
So if you do BE: locked_pending = 1, you set bytes[1], not bytes[0].


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