Date: Mon, 3 Mar 2008 12:11:12 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
cc: Pavel Machek <pavel@ucw.cz>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@osdl.org>,
       Zdenek Kabelac <zdenek.kabelac@gmail.com>, <davem@davemloft.net>,
       "Rafael J. Wysocki" <rjw@sisk.pl>, Pierre Ossman <drzeus-mmc@drzeus.cx>,
       Kernel development list <linux-kernel@vger.kernel.org>,
       pm list <linux-pm@lists.linux-foundation.org>
Subject: Re: [patch] Re: using long instead of atomic_t when only set/read
 is required
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Message-ID: <Pine.LNX.4.44L0.0803031207120.3611-100000@iolanthe.rowland.org>
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On Mon, 3 Mar 2008, Alan Cox wrote:

> > Atomicity of reads of write for pointers and integral types (other than 
> > long long) should be documented.
> 
> NAK.
> 
> Atomicity of reads or writes for pointers and integral types is NOT
> guaranteed. Gcc doesn't believe in your guarantee.

Miscommunication and lack of clarity.  CPU reads and writes _are_ 
guaranteed to be atomic.  What is not guaranteed is that the compiler 
will generate a single read or write instruction corresponding to a 
particular expression in C.

Consider a routine like the following:

	static task_struct *the_task;

	void store_task(void)
	{
		the_task = current;
	}

Is it possible to say whether readers examining "the_task" are 
guaranteed to see a coherent value?

Alan Stern		

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