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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Subject: Re: [patch] Re: using long instead of atomic_t when only set/read is required
Date: Tue, 4 Mar 2008 04:16:33 +1100
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Cc: Alan Stern <stern@rowland.harvard.edu>, 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>,
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       pm list <linux-pm@lists.linux-foundation.org>
References: <20080303120842.GA28369@elf.ucw.cz> <Pine.LNX.4.44L0.0803031023550.3611-100000@iolanthe.rowland.org> <20080303155330.39e45ad4@core>
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On Tuesday 04 March 2008 02:53, 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.

Are you sure gcc doesn't? Or is it just "C"?

Linux wouldn't work today if gcc did something non-atomic there
(presuming you're talking about naturally aligned pointers/ints).
It is widely used and accepted.

RCU users are far from the only places to rely on this, although
I guess they are the main ones when it comes to assigning pointers
atomically.

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