Re: [patch] Re: using long instead of atomic_t when only set/read is required

Alan Stern
2008-03-03T15:42:45Z

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@suse.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@rowland.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.