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Date: Mon, 3 Mar 2008 09:26:24 -0800 (PST)
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Alan Stern <stern@...land.harvard.edu>
cc: Alan Cox <alan@...rguk.ukuu.org.uk>, Pavel Machek <pavel@....cz>,
"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
On Mon, 3 Mar 2008, Alan Stern wrote:
>
> 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).
So in that sense, the atomicity guarantees are a lot weaker than the ones
we do for IO accesses, but that's all fine. Memory isn't IO, and doesn't
have side effects.
Linus
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