Message-ID: <53504BB3.4010009@hp.com>
Date: Thu, 17 Apr 2014 17:46:27 -0400
From: Waiman Long <waiman.long@hp.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-arch@vger.kernel.org,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        xen-devel@lists.xenproject.org, kvm@vger.kernel.org,
        Paolo Bonzini <paolo.bonzini@gmail.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Rik van Riel <riel@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        David Vrabel <david.vrabel@citrix.com>,
        Oleg Nesterov <oleg@redhat.com>, Gleb Natapov <gleb@redhat.com>,
        Scott J Norton <scott.norton@hp.com>, Chegu Vinod <chegu_vinod@hp.com>
Subject: Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS
References: <1397747051-15401-1-git-send-email-Waiman.Long@hp.com> <1397747051-15401-6-git-send-email-Waiman.Long@hp.com> <20140417155649.GR11096@twins.programming.kicks-ass.net>
In-Reply-To: <20140417155649.GR11096@twins.programming.kicks-ass.net>
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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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