Re: [patch V2 1/8] signal: Prevent exec() race

[Frederic Weisbecker]

Le Fri, Sep 11, 2026 at 12:22:03PM +0200, Peter Zijlstra a écrit :
> On Fri, Sep 11, 2026 at 11:58:04AM +0200, Frederic Weisbecker wrote:
> > Le Thu, Sep 10, 2026 at 03:28:43PM +0200, Peter Zijlstra a écrit :
> 
> > > The way data dependencies work in this case is a LOAD->LOAD ordering.
> > > The LOAD-ACQUIRE that is part of LOCK must happen after the initial
> > > load. And then the later load is constrained by the ACQUIRE.
> > > 
> > > So LOAD(B) must resolve in order to do LOAD_ACQUIRE(B->lock.value).
> > 
> > Ok I must confess that's not a pattern I'm used to and therefore it's
> > still a bit counter-intuitive to me. I hope we can document that somehow
> > somewhere.
> 
> One way of looking at it is the computation for the address of
> &B->lock.value.
> 
> A	void *r = LOAD(B);			 // B
> B	r += offsetof(typeof(*B), lock);	 // &B->lock
> 	spin_lock(r)
> C	  r += offsetof(typeof(B->lock), value); // &B->lock.value
> 	  for (;;)
> D	    v = LOAD_ACQUIRE(r);
> 	    if (!v && !cmpxchg_relaxed(r, 0, 1))
> 	      break;
> 	    cpu_relax();
> 
> Note how the LOAD_ACQUIRE() at D depends on the computation of C, and B,
> which in turn depend on the load of A.
> 
> If A does not complete, B,C cannot compute the address for the load of
> D. Therefore A must come before D.

Right, it's just that acquire is often described as ordering access after
a single memory target (matching release before that same single memory target).
But I'm discovering that this property also applies to a memory target that
depends on an actual target, so to speak :o)

Anyway yes, I get the idea now, thanks for the patient explanations!