linux-kernel - Re: How does spin_unlock() in x86-64 align with the description in Documention/memory-barriers.txt?

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Date:	Fri, 22 Mar 2013 12:15:09 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Zhu Yanhai" <zhu.yanhai@...il.com>
Cc:	<x86@...nel.org>, "Thomas Gleixner" <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Richard Weinberger" <richard@....at>,
	"Ingo Molnar" <mingo@...hat.com>, <muming.wq@...bao.com>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: How does spin_unlock() in x86-64 align with the
 description in Documention/memory-barriers.txt?

>>> On 22.03.13 at 12:58, Zhu Yanhai <zhu.yanhai@...il.com> wrote:
> Hi all,
> In the documention it reads,
> 
> (2) UNLOCK operation implication:
> 
>      Memory operations issued before the UNLOCK will be completed before the
>      UNLOCK operation has completed.
> 
>      Memory operations issued after the UNLOCK may be completed before the
>      UNLOCK operation has completed.
> 
> However, on x86-64 __ticket_spin_unlock() merely does,
> 
> static __always_inline void __ticket_spin_unlock(raw_spinlock_t *lock)
> {
>     asm volatile(
>         ALTERNATIVE(UNLOCK_LOCK_PREFIX"incb (%0);"ASM_NOP3,
>                 UNLOCK_LOCK_ALT_PREFIX"movw $0, (%0)",
>                 X86_FEATURE_UNFAIR_SPINLOCK)
>         :
>         : "Q" (&lock->slock)
>         : "memory", "cc");
> }
> 
> While both UNLOCK_LOCK_PREFIX and UNLOCK_LOCK_ALT_PREFIX are empty
> strings. So how such a function keeps the memory operations issued
> before it completed?

Please read the section "Memory Ordering in P6 and More Recent
Processor Families" in SDM Vol 3.

Jan

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