On Tue, Sep 08, 2026 at 11:22:14AM -0400, Mathieu Desnoyers 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.
>
> 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);
> --
> 2.43.0
>
Applied this fix patch on top of Pual's v2 RFC patch series. The issue
cannot be reproduce.
Tested-by: kernel test robot <[email protected]>
