From: Linus Torvalds <torvalds@linux-foundation.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, Pavel Machek <pavel@ucw.cz>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Andrew Morton <akpm@linux-foundation.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
Date: Mon, 3 Mar 2008 09:26:24 -0800 (PST) [thread overview]
Message-ID: <alpine.LFD.1.00.0803030921420.17889@woody.linux-foundation.org> (raw)
In-Reply-To: <Pine.LNX.4.44L0.0803031207120.3611-100000@iolanthe.rowland.org>
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
next prev parent reply other threads:[~2008-03-03 17:38 UTC|newest]
Thread overview: 93+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20080219232338.E9A24108068@picon.linux-foundation.org>
2008-02-20 16:42 ` [Bug 10030] Suspend doesn't work when SD card is inserted Alan Stern
2008-02-20 17:30 ` Pierre Ossman
2008-02-20 19:26 ` Alan Stern
2008-02-20 20:51 ` Pierre Ossman
2008-02-20 20:58 ` Rafael J. Wysocki
2008-02-20 21:06 ` Alan Stern
2008-02-20 22:15 ` Rafael J. Wysocki
2008-02-20 22:24 ` Alan Stern
2008-02-20 22:41 ` Rafael J. Wysocki
2008-02-21 0:02 ` Rafael J. Wysocki
2008-02-21 16:27 ` Alan Stern
2008-02-21 16:38 ` Rafael J. Wysocki
2008-02-21 17:48 ` Alan Stern
2008-02-21 22:47 ` Rafael J. Wysocki
2008-02-21 23:05 ` Alan Stern
2008-02-23 1:30 ` Rafael J. Wysocki
2008-02-23 4:39 ` Alan Stern
2008-02-23 20:16 ` Rafael J. Wysocki
2008-02-23 23:29 ` Alan Stern
2008-02-24 0:19 ` Rafael J. Wysocki
2008-02-24 3:25 ` Alan Stern
2008-02-24 4:26 ` [linux-pm] " Alan Stern
2008-02-24 14:00 ` Rafael J. Wysocki
2008-02-24 15:33 ` Bugs in MMC [was: [Bug 10030] Suspend doesn't work when SD card is inserted] Alan Stern
2008-02-25 17:41 ` Pierre Ossman
2008-02-25 17:58 ` Alan Stern
2008-02-25 18:31 ` Pierre Ossman
2008-02-25 20:00 ` Alan Stern
2008-03-01 14:11 ` Pierre Ossman
2008-03-01 14:36 ` Alan Stern
2008-03-01 14:47 ` Pierre Ossman
2008-02-25 22:51 ` Felipe Balbi
2008-03-03 21:59 ` David Brownell
2008-03-04 6:03 ` Pierre Ossman
2008-03-04 9:44 ` David Brownell
2008-03-04 9:58 ` Pierre Ossman
2008-03-06 21:23 ` David Brownell
2008-03-04 17:53 ` Alan Stern
2008-03-04 18:53 ` David Brownell
2008-03-04 19:51 ` Alan Stern
2008-03-04 20:30 ` David Brownell
2008-03-04 21:00 ` Alan Stern
2008-03-06 15:55 ` Pavel Machek
2008-03-06 20:33 ` Alan Stern
2008-03-06 20:53 ` Zdenek Kabelac
2008-03-06 21:31 ` Rafael J. Wysocki
2008-03-04 17:50 ` Alan Stern
2008-02-24 13:33 ` [Bug 10030] Suspend doesn't work when SD card is inserted Rafael J. Wysocki
2008-02-24 20:25 ` Rafael J. Wysocki
2008-02-24 20:45 ` Alan Stern
2008-02-24 20:56 ` Rafael J. Wysocki
2008-02-24 21:11 ` Alan Stern
2008-02-24 22:18 ` Rafael J. Wysocki
2008-02-24 13:51 ` Rafael J. Wysocki
2008-02-24 19:27 ` Alan Stern
2008-02-24 19:42 ` Zdenek Kabelac
2008-02-24 20:09 ` Rafael J. Wysocki
2008-02-24 18:21 ` Pavel Machek
2008-02-24 19:03 ` Alan Stern
2008-02-24 20:11 ` Pavel Machek
2008-02-24 20:33 ` Alan Stern
2008-02-24 21:42 ` Pavel Machek
2008-02-24 22:21 ` Rafael J. Wysocki
2008-02-25 2:21 ` Alan Stern
2008-02-25 11:41 ` Rafael J. Wysocki
[not found] ` <c4e36d110802250402q7312a488idf752e07db7504e8@mail.gmail.com>
2008-02-25 12:31 ` Rafael J. Wysocki
2008-02-25 2:19 ` Alan Stern
2008-02-25 9:03 ` using long instead of atomic_t when only set/read is required (was Re: [Bug 10030] Suspend doesn't work when SD card is inserted) Pavel Machek
2008-02-25 14:46 ` Alan Stern
2008-03-03 12:08 ` [patch] Re: using long instead of atomic_t when only set/read is required Pavel Machek
2008-03-03 15:42 ` Alan Stern
2008-03-03 15:53 ` Alan Cox
2008-03-03 17:11 ` Alan Stern
2008-03-03 17:26 ` Linus Torvalds [this message]
2008-03-03 17:44 ` Pavel Machek
2008-03-03 19:27 ` Alan Stern
2008-03-06 15:58 ` Mark Lord
2008-03-06 16:11 ` Linus Torvalds
2008-03-06 16:27 ` Mark Lord
2008-03-03 17:16 ` Nick Piggin
2008-03-03 17:31 ` Paul E. McKenney
2008-03-03 17:33 ` Alan Cox
2008-03-03 17:22 ` Paul E. McKenney
2008-03-03 15:48 ` Alan Cox
2008-03-03 17:24 ` Pavel Machek
2008-03-03 20:27 ` Rafael J. Wysocki
2008-03-03 21:12 ` Paul E. McKenney
2008-03-03 22:23 ` Linus Torvalds
2008-03-04 23:32 ` Peter Hartley
2008-03-04 23:54 ` Rafael J. Wysocki
2008-03-05 0:26 ` Paul E. McKenney
2008-02-25 11:40 ` [Bug 10030] Suspend doesn't work when SD card is inserted Rafael J. Wysocki
2008-03-05 17:07 [patch] Re: using long instead of atomic_t when only set/read is required Matti Linnanvuori
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