On Tue, Mar 10, 2026 at 10:41:46PM +0100, Sabrina Dubroca wrote:

quoted

Yes, this is possible scenario and this is what is worth to document.

We could add something like:

/* Take a reference to @x, when we know the state has a refcount >= 1.
 * In this case, we can avoid refcount_inc_not_zero and the error
 * handling it requires.
 * In contexts where concurrent state deletion is possible and we
 * don't already hold a reference to that state, xfrm_state_hold_rcu
 * must be used.
 */

Though it may not make much sense to refer to xfrm_state_hold_rcu
(implemented in net/xfrm/xfrm_state.c) from include/net/xfrm.h.

And if we consider the hashtables to be private to
net/xfrm/xfrm_state.c, nothing outside of it should ever see a state
with refcount=0, since they will only ever see states that already
have one reference held by whatever gave them the pointer.

So maybe it's more xfrm_state_hold_rcu that needs a mention of
"concurrent state deletion could bring the refcount to 0 while we're
doing the lookup"? I don't know, for me it's pretty obvious with the
_rcu suffix that RCU -> unlocked -> could be deleted in parallel.


We also have __xfrm_state_put, the commit message that introduced it
states:

    We often just do an atomic_dec(&x->refcnt) on an xfrm_state object
    because we know there is more than 1 reference remaining and thus
    we can elide the heavier xfrm_state_put() call.

so we could add:

/* Drop a reference to @x, when we know there is more than 1 reference remaining.
 * In this case, we can avoid refcount_dec_and_test and just decrement refcnt.
 */

but maybe someone has a better suggestion.

I plan to take this series as is. If you feel we need a comment,
just add it with a followup patch.