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

Pavel Machek
2008-03-03T17:44:36Z

Hi!

> > 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).

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.

Pavel
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