On Tue, Sep 08, 2026 at 11:29:31AM -0400, Mathieu Desnoyers wrote:
> On 2026-09-08 11:24, Bradley Morgan wrote:
> > On 8 September 2026 16:22:14 BST, Mathieu Desnoyers
> > <[email protected]> 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 ?
I am fine either way, as long as the information is reasonably easily
accessible. Which is the case either way. ;-)
Thanx, Paul
> Thanks,
>
> Mathieu
>
> >
> >
> > > Fixes: 6357ec235c59 ("hazptrtorture: Fix hazptr ownership issue")
> > > Reported-by: kernel test robot <[email protected]>
> > > Closes: https://lore.kernel.org/oe-lkp/[email protected]
> > > Signed-off-by: Mathieu Desnoyers <[email protected]>
> > > Reviewed-by: Bradley Morgan <[email protected]>
> > > Cc: Paul E. McKenney <[email protected]>
> > > Cc: Boqun Feng <[email protected]>
> > > Cc: Bradley Morgan <[email protected]>
> > > Cc: <[email protected]>
> > > Cc: <[email protected]>
> > > ---
> > > 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/[email protected]/
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
