Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address dependency

[Jonas Oberhauser]

Am 9/29/2024 um 12:26 AM schrieb Alan Huang:
> 2024年9月28日 23:55，Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote：
> > On 2024-09-28 17:49, Alan Stern wrote:
> > > On Sat, Sep 28, 2024 at 11:32:18AM -0400, Mathieu Desnoyers wrote:
> > > > On 2024-09-28 16:49, Alan Stern wrote:
> > > > > On Sat, Sep 28, 2024 at 09:51:27AM -0400, Mathieu Desnoyers wrote:
> > > > > > equality, which does not preserve address dependencies and allows the
> > > > > > following misordering speculations:
> > > > > >
> > > > > > - If @b is a constant, the compiler can issue the loads which depend
> > > > > >     on @a before loading @a.
> > > > > > - If @b is a register populated by a prior load, weakly-ordered
> > > > > >     CPUs can speculate loads which depend on @a before loading @a.
> > > > >
> > > > > It shouldn't matter whether @a and @b are constants, registers, or
> > > > > anything else.  All that matters is that the compiler uses the wrong
> > > > > one, which allows weakly ordered CPUs to speculate loads you wouldn't
> > > > > expect it to, based on the source code alone.
> > > >
> > > > I only partially agree here.
> > > >
> > > > On weakly-ordered architectures, indeed we don't care whether the
> > > > issue is caused by the compiler reordering the code (constant)
> > > > or the CPU speculating the load (registers).
> > > >
> > > > However, on strongly-ordered architectures, AFAIU, only the constant
> > > > case is problematic (compiler reordering the dependent load), because
> > > I thought you were trying to prevent the compiler from using one pointer
> > > instead of the other, not trying to prevent it from reordering anything.
> > > Isn't this the point the documentation wants to get across when it says
> > > that comparing pointers can be dangerous?
> >
> > The motivation for introducing ptr_eq() is indeed because the
> > compiler barrier is not sufficient to prevent the compiler from
> > using one pointer instead of the other.
>
> barrier_data(&b) prevents that.

I don't think one barrier_data can garantuee preventing this, because 
right after doing the comparison, the compiler still could do b=a.

In that case you would be guaranteed to use the value in b, but that 
value is not the value loaded into b originally but rather the value 
loaded into a, and hence your address dependency goes to the wrong load 
still.

However, doing

barrier_data(&b);
if (a == b) {
   barrier();
   foo(*b);
}

might maybe prevent it, because after the address of b is escaped, the 
compiler might no longer be allowed to just do b=a;, but I'm not sure if 
that is completely correct, since the compiler knows b==a and no other 
thread can be concurrently modifying a or b. Therefore, given that the 
compiler knows the hardware, it might know that assigning b=a would not 
cause any  race-related issues even if another thread was reading b 
concurrently.

Finally, it may be only a combination of barrier_data and making b 
volatile could be guaranteed to solve the issue, but the code will be 
very obscure compared to using ptr_eq.

  jonas