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SubjectRe: [patch] Re: using long instead of atomic_t when only set/read is required
On Mon, 3 Mar 2008, Pavel Machek wrote:

> Ok... can we get Alan Stern's patch into Documentation/atomic_ops.txt
> , then? I was not aware of this, and there seems to be lot of
> confusion around...
> Plus... I really don't think we can "just access" this as normal
> pointers... due to the compiler issues Alan Cox mentioned, and due to
> the ACCESS_ONCE() issue.

Here's an updated version of the patch, including the issue Alan Cox
brought up.

Alan Stern


Atomicity of reads of write for pointers and integral types (other than
long long) should be documented, along with the limitations imposed by
the compiler.

Signed-off-by: Alan Stern <>


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 +21,24 @@ 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! Note also that gcc does not
+guarantee to compile all C assignment expressions into simple writes.
+For example, a statement like "x = a + b" might cause gcc to emit code
+equivalent to "x = a; x += b", which is decidedly non-atomic.
The first operations to implement for atomic_t's are the initializers and
plain reads.

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