Re: [PATCH] hazptr: handle NULL address in hazptr_detach

[Mathieu Desnoyers]

On 2026-09-08 11:24, Bradley Morgan wrote:
> On 8 September 2026 16:22:14 BST, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > When hazptr_acquire loads a NULL pointer, it sets:
> >
> > - slot_item->slot.addr = NULL,
> > - slot_item->ctx.ctx = ctx
> > - ctx->slot = slot
> >
> > And it returns NULL.
> >
> > Then hazptr_detach is called on this ctx, it will act on the ctx as if
> > needed to be promoted to backup slot, even though it has a NULL addr.
> >
> > Looking at what hazptr_note_context_switch() does before promoting
> > to backup slot, it checks for a NULL slot->addr, which is exactly
> > what is missing from hazptr_detach.
> >
> > With this in place there would be no need to explicitly check the
> > hazptr_acquire() return value before calling hazptr_detach().
> >
> > hazptr_release() has a early return check for NULL addr as well, so it
> > makes sense that detach does an early return (no-op) similarly.
>
> You shall kill me for this!!
>
> Could you perhaps do a splat in ze commit description pls?

The splat is available at the "Closes" URL below. I'm not sure whether
we should duplicate this verbose information ?

Paul ?

Thanks,

Mathieu

> > Fixes: 6357ec235c59 ("hazptrtorture: Fix hazptr ownership issue")
> > Reported-by: kernel test robot <yi1.lai@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-lkp/202608130915.62b53936-lkp@xxxxxxxxx
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Boqun Feng <boqun@xxxxxxxxxx>
> > Cc: Bradley Morgan <brads@xxxxxxxxxxxxxx>
> > Cc: <rcu@xxxxxxxxxxxxxxx>
> > Cc: <lkmm@xxxxxxxxxxxxxxx>
> > ---
> > include/linux/hazptr.h | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/hazptr.h b/include/linux/hazptr.h
> > index 43122c5673bd..d1670121947a 100644
> > --- a/include/linux/hazptr.h
> > +++ b/include/linux/hazptr.h
> > @@ -160,10 +160,12 @@ void hazptr_detach(struct hazptr_ctx *ctx)
> > 	struct hazptr_slot *slot;
> >
> > 	guard(preempt)();
> > +	slot = ctx->slot;
> > +	if (!slot->addr)
> > +		return;
> > #ifdef CONFIG_HAZPTR_DEBUG
> > 	ctx->detach_task = ctx->detach_cpu = true;
> > #endif
> > -	slot = ctx->slot;
> > 	if (unlikely(hazptr_slot_is_backup(ctx, slot)))
> > 		return;
> > 	hazptr_promote_to_backup_slot(ctx, slot);
>
> --- Thanks!
> https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@xxxxxxxxx/


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com