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

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Date:	Thu, 17 Apr 2014 17:56:49 +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 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.
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