linux-kernel - Re: [patch] Re: using long instead of atomic

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Date:	Mon, 3 Mar 2008 18:44:22 +0100
From:	Pavel Machek <pavel@....cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zdenek Kabelac <zdenek.kabelac@...il.com>, davem@...emloft.net,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Pierre Ossman <drzeus-mmc@...eus.cx>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [patch] Re: using long instead of atomic_t when only set/read
	is required

Hi!

> > 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?
> 
> Yes, we do depend on this.  All the RCU stuff (and in general *anything* 
> that depends on memory ordering as opposed to full locking, and we have 
> quite a lot of it) is very fundamentally dependent on the fact that things 
> like pointers get read and written atomically.
> 
> HOWEVER, it is worth pointing out that it's generally true in a 
> "different" sense than the actual atomic accesses. For example, if you 
> test a single bit of a word, it's still quite possible that gcc will have 
> turned that "atomic" read into a single byte read, so it's not necessarily 
> the case that we'll actually even read the whole word. 
> 
> (Writes are different: if you do things like bitwise updates they simply 
> *will*not* be atomic, but that's simply not what we depend on anyway).

Ok... can we get Alan Stern's patch into Documentation/atomic_ops.txt
, then? I was not aware of this, and there seems to be lot of
confusion around...

Plus... I really don't think we can "just access" this as normal
pointers... due to the compiler issues Alan Cox mentioned, and due to
the ACCESS_ONCE() issue.

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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