Re: [RFC][PATCH 0/5] arch: atomic rework

[Peter Zijlstra <peterz at infradead dot org>]

On Tue, Feb 18, 2014 at 12:12:06PM +0000, Peter Sewell wrote:
> Several of you have said that the standard and compiler should not
> permit speculative writes of atomics, or (effectively) that the
> compiler should preserve dependencies.

The example below only deals with control dependencies; so I'll limit
myself to that.

> In simple examples it's easy
> to see what that means, but in general it's not so clear what the
> language should guarantee, because dependencies may go via non-atomic
> code in other compilation units, and we have to consider the extent to
> which it's desirable to limit optimisation there.
> 
> For example, suppose we have, in one compilation unit:
> 
>     void f(int ra, int*rb) {
>       if (ra==42)
>         *rb=42;
>       else
>         *rb=42;
>     }
> 
> and in another compilation unit the bodies of two threads:
> 
>     // Thread 0
>     r1 = x;
>     f(r1,&r2);
>     y = r2;
> 
>     // Thread 1
>     r3 = y;
>     f(r3,&r4);
>     x = r4;
> 
> where accesses to x and y are annotated C11 atomic
> memory_order_relaxed or Linux ACCESS_ONCE(), accesses to
> r1,r2,r3,r4,ra,rb are not annotated, and x and y initially hold 0.

So I'm intuitively ok with this, however I would expect something like:

  void f(_Atomic int ra, _Atomic int *rb);

To preserve dependencies and not make the conditional go away, simply
because in that case the:

  if (ra == 42)

the 'ra' usage can be seen as an atomic load.

> So as far as we can see, either:
> 
> 1) if you can accept the latter behaviour (if the Linux codebase does
>    not rely on its absence), the language definition should permit it,
>    and current compiler optimisations can be used,

Currently there's exactly 1 site in the Linux kernel that relies on
control dependencies as far as I know -- the one I put in. And its
limited to a single function, so no cross translation unit funnies
there.

Of course, nobody is going to tell me when or where they'll put in the
next one; since its now documented as accepted practise.

However, PaulMck and our RCU usage very much do cross all sorts of TU
boundaries; but those are data dependencies.

~ Peter