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

[kernel test robot <yi1.lai@intel.com>]

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 <yi1.lai@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202608130915.62b53936-lkp@intel.com
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Reviewed-by: Bradley Morgan <brads@mainlining.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Boqun Feng <boqun@kernel.org>
> Cc: Bradley Morgan <brads@mainlining.org>
> Cc: <rcu@vger.kernel.org>
> Cc: <lkmm@lists.linux.dev>
> ---
>  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 <yi1.lai@intel.com>