[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Mon, 3 Mar 2008 10:42:35 -0500 (EST)
From: Alan Stern <stern@...land.harvard.edu>
To: Pavel Machek <pavel@....cz>
cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...l.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, Pavel Machek wrote:
> Hi!
>
> > > Alan thinks that `subj` is correct...
> >
> > More precisely, reads and writes of pointers are always atomic. That
> > is, if a write and a read occur concurrently, it is guaranteed that the
> > read will obtain either the old or the new value of the pointer, never
> > a mish-mash of the two. If this were not so then RCU wouldn't work.
Right, Paul?
> Ok, so linux actually atomicity of long?
>
> If so, this should probably be applied...
>
> Signed-off-by: Pavel Machek <pavel@...e.cz>
>
> diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
> index 4ef2450..0a7d180 100644
> --- a/Documentation/atomic_ops.txt
> +++ b/Documentation/atomic_ops.txt
> @@ -21,6 +21,9 @@ local_t is very similar to atomic_t. If
> updated by one CPU, local_t is probably more appropriate. Please see
> Documentation/local_ops.txt for the semantics of local_t.
>
> +long (and int and void *) can be used instead of atomic_t, if all you
> +need is atomic setting and atomic reading.
> +
> The first operations to implement for atomic_t's are the initializers and
> plain reads.
Yes indeed. This fact doesn't seem to be documented anywhere, but it
is clearly a requirement of the kernel. I would make the text a little
more explicit, see below.
Alan Stern
-------------------------------------------------------
Atomicity of reads of write for pointers and integral types (other than
long long) should be documented.
Signed-off-by: Alan Stern <stern@...land.harvard.edu>
---
Index: usb-2.6/Documentation/atomic_ops.txt
===================================================================
--- usb-2.6.orig/Documentation/atomic_ops.txt
+++ usb-2.6/Documentation/atomic_ops.txt
@@ -21,6 +21,21 @@ local_t is very similar to atomic_t. If
updated by one CPU, local_t is probably more appropriate. Please see
Documentation/local_ops.txt for the semantics of local_t.
+For all properly-aligned pointer and integral types other than long
+long, the kernel requires simple reads and writes to be atomic with
+respect to each other. That is, if one CPU reads a pointer value at
+the same time as another CPU overwrites the pointer, it is guaranteed
+that the reader will obtain either the old or the new value of the
+pointer, never some mish-mash combination of the two. Likewise, if
+one CPU writes a long value at the same time as another CPU does, it
+is guaranteed that one or the other of the values will end up stored
+in memory, not some mish-mash combination of bits.
+
+Thus, if all you need is atomicity of reading and writing then you can
+use plain old ints, longs, or pointers; you don't need to use
+atomic_t. (But note: This guarantee emphatically does not apply to
+long long values or unaligned values!)
+
The first operations to implement for atomic_t's are the initializers and
plain reads.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists