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

[Alan Stern <stern@rowland.harvard.edu>]

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 <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 +21,24 @@ 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.