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

Linus Torvalds
2008-03-03T17:38:39Z

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