Re: [PATCH] refcount: Strengthen inc_not_zero()

[Will Deacon]

On Wed, Jan 15, 2025 at 05:00:11PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 15, 2025 at 12:13:34PM +0100, Peter Zijlstra wrote:
> 
> > Notably, it means refcount_t is entirely unsuitable for anything
> > SLAB_TYPESAFE_BY_RCU, since they all will need secondary validation
> > conditions after the refcount succeeds.
> > 
> > And this is probably fine, but let me ponder this all a little more.
> 
> Even though SLAB_TYPESAFE_BY_RCU is relatively rare, I think we'd better
> fix this, these things are hard enough as they are.
> 
> Will, others, wdyt?

We should also update the Documentation (atomic_t.txt and
refcount-vs-atomic.rst) if we strengthen this.

> ---
> Subject: refcount: Strengthen inc_not_zero()
> 
> For speculative lookups where a successful inc_not_zero() pins the
> object, but where we still need to double check if the object acquired
> is indeed the one we set out to aquire, needs this validation to happen
> *after* the increment.
> 
> Notably SLAB_TYPESAFE_BY_RCU is one such an example.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  include/linux/refcount.h | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> index 35f039ecb272..340e7ffa445e 100644
> --- a/include/linux/refcount.h
> +++ b/include/linux/refcount.h
> @@ -69,9 +69,10 @@
>   * its the lock acquire, for RCU/lockless data structures its the dependent
>   * load.
>   *
> - * Do note that inc_not_zero() provides a control dependency which will order
> - * future stores against the inc, this ensures we'll never modify the object
> - * if we did not in fact acquire a reference.
> + * Do note that inc_not_zero() does provide acquire order, which will order
> + * future load and stores against the inc, this ensures all subsequent accesses
> + * are from this object and not anything previously occupying this memory --
> + * consider SLAB_TYPESAFE_BY_RCU.
>   *
>   * The decrements will provide release order, such that all the prior loads and
>   * stores will be issued before, it also provides a control dependency, which
> @@ -144,7 +145,7 @@ bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
>  	do {
>  		if (!old)
>  			break;
> -	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
> +	} while (!atomic_try_cmpxchg_acquire(&r->refs, &old, old + i));

Hmm, do the later memory accesses need to be ordered against the store
part of the increment or just the read? If it's the former, then I don't
think that _acquire is sufficient -- accesses can still get in-between
the read and write parts of the RmW.

Will