linux-kernel - Re: why are some atomic_t's not volatile, while most are?

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Message-ID: <46B8E1D3.8050501@redhat.com>
Date:	Tue, 07 Aug 2007 17:19:15 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Chris Friesen <cfriesen@...tel.com>
CC:	Jerry Jiang <wjiang@...ilience.com>,
	"Robert P. J. Day" <rpjday@...dspring.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why are some atomic_t's not volatile, while most are?

Chris Friesen wrote:
> Chris Snook wrote:
> 
>> But if you're not using SMP, the only way you get a race condition is 
>> if your compiler is reordering instructions that have side effects 
>> which are invisible to the compiler.  This can happen with MMIO 
>> registers, but it's not an issue with an atomic_t we're declaring in 
>> real memory.
> 
> I refer back to the interrupt handler case.  Suppose we have:
> 
> while(!atomic_read(flag))
>      continue;
> 
> where flag is an atomic_t that is set in an interrupt handler, the 
> volatile may be necessary on some architectures to force the compiler to 
> re-read "flag" each time through the loop.
> 
> Without the "volatile", the compiler could be perfectly within its 
> rights to evaluate "flag" once and create an infinite loop.
> 
> Now I'm not trying to say that we should explictly use "volatile" in 
> common code, but that it is possible that it is required within the 
> arch-specific atomic_t accessors even on uniprocessor systems.
> 
> Chris

That's why we define atomic_read like so:

#define atomic_read(v)          ((v)->counter)

This avoids the aliasing problem, because the compiler must de-reference the 
pointer every time, which requires a memory fetch.  This is usually fast thanks 
to caching, and hardware cache invalidation enforces correctness when it does 
change.

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