linux-kernel - Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

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Message-Id: <200905221503.08586.mszick@morethan.org>
Date:	Fri, 22 May 2009 15:03:06 -0500
From:	"Michael S. Zick" <mszick@...ethan.org>
To:	Roland Dreier <rdreier@...co.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

On Fri May 22 2009, Roland Dreier wrote:
> 
>  > Unless you have interrupts enabled, then you have two contexts.
>  > Only xchg is "naturally" atomic.
> 
> Isn't the lock prefix about consistency between multiple processors?
> The x86 architecture always handles interrupts on instruction
> boundaries.  I'm guessing you're worried about definitions like
> 
> static inline void atomic_inc(atomic_t *v)
> {
> 	asm volatile(LOCK_PREFIX "incl %0"
> 		     : "+m" (v->counter));
> }
> 
> which compiles to just "incl" (with no lock prefix) on uniprocessor
> kernels; but the IA-32 architecture guarantees that the incl instruction
> cannot be interrupted between reading the old value and writing the new
> value.
> 

Not prior to P-4, and since then only "may" be done atomically,
see reference post in my earlier reply.

PS: And yes, that was where I spotted the usage first.  ;)

Mike
>  - R.
> 
> 

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