Re: [PATCH 19/28] hazptr: Permit detaching hazard pointers from contexts

[Boqun Feng <boqun@kernel.org>]

On Fri, Sep 18, 2026 at 05:00:47PM -0700, Paul E. McKenney wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> 
> Provide a new hazptr_detach() function that detaches a given hazard
> pointer from its acquisition context.  This context might be a task or
> an interrupt handler.
> 
> [ paulmck: s/hazptr_detach_from_task/hazptr_detach/ per Mathieu. ]
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Boqun Feng <boqun@kernel.org>
> Cc: <rcu@vger.kernel.org>
> Cc: <lkmm@lists.linux.dev>
> ---
>  include/linux/hazptr.h | 55 ++++++++++++++++++++++++++++--------------
>  1 file changed, 37 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/hazptr.h b/include/linux/hazptr.h
> index b121f7779cda..8197a51b9f7a 100644
> --- a/include/linux/hazptr.h
> +++ b/include/linux/hazptr.h
> @@ -98,6 +98,41 @@ bool hazptr_slot_is_backup(struct hazptr_ctx *ctx, struct hazptr_slot *slot)
>  	return slot == &ctx->backup_slot.slot;
>  }
>  
> +/* Internal helper. */
> +static inline
> +void hazptr_promote_to_backup_slot(struct hazptr_ctx *ctx, struct hazptr_slot *slot)
> +{
> +	struct hazptr_slot *backup_slot;
> +
> +	backup_slot = hazptr_chain_backup_slot(ctx);
> +	/*
> +	 * Move hazard pointer from the per-CPU slot to the
> +	 * backup slot. This requires hazard pointer
> +	 * synchronize to iterate on per-CPU slots with
> +	 * load-acquire before iterating on the overflow list.
> +	 */
> +	WRITE_ONCE(backup_slot->addr, slot->addr);
> +	/*
> +	 * store-release orders store to backup slot addr before
> +	 * store to per-CPU slot addr.
> +	 */
> +	smp_store_release(&slot->addr, NULL);
> +	/* Use the backup slot for context. */
> +	ctx->slot = backup_slot;
> +}
> +

Probably needs a function doc for hazptr_detach(), how about the
following?

/*
 * hazptr_detach: Detach the hazptr from its acquisition context.
 *
 * By default hazptr_acquire() expects to be released from its
 * acquisition context. However there are cases where the acquisition
 * context sends (in other words, transfers the ownership of) the hazptr
 * to another context, e.g.
 * 
 * 	hazptr_acquire(ctx, gp);
 * 	hazptr_detach(ctx);
 * 	defer_work->hp = ctx;
 * 	queue_work(..., defer_work);
 * 	<in defer_work->func()>
 * 	hazptr_release(ctx);
 *
 * Use hazptr_detach() before sending the hazptr across contexts.
 *
 * It's safe to detach an "already-detatched" hazptr.
 */

Thoughts?

Regards,
Boqun

> +static inline
> +void hazptr_detach(struct hazptr_ctx *ctx)
> +{
> +	struct hazptr_slot *slot;
> +
> +	guard(preempt)();
> +	slot = ctx->slot;
> +	if (unlikely(hazptr_slot_is_backup(ctx, slot)))
> +		return;
> +	hazptr_promote_to_backup_slot(ctx, slot);
> +}
> +
>  static inline
>  void hazptr_note_context_switch(void)
>  {
> @@ -106,27 +141,11 @@ void hazptr_note_context_switch(void)
>  
>  	for (idx = 0; idx < NR_HAZPTR_PERCPU_SLOTS; idx++) {
>  		struct hazptr_slot_item *item = &percpu_slots->items[idx];
> -		struct hazptr_slot *slot = &item->slot, *backup_slot;
> -		struct hazptr_ctx *ctx;
> +		struct hazptr_slot *slot = &item->slot;
>  
>  		if (!slot->addr)
>  			continue;
> -		ctx = item->ctx.ctx;
> -		backup_slot = hazptr_chain_backup_slot(ctx);
> -		/*
> -		 * Move hazard pointer from the per-CPU slot to the
> -		 * backup slot. This requires hazard pointer
> -		 * synchronize to iterate on per-CPU slots with
> -		 * load-acquire before iterating on the overflow list.
> -		 */
> -		WRITE_ONCE(backup_slot->addr, slot->addr);
> -		/*
> -		 * store-release orders store to backup slot addr before
> -		 * store to per-CPU slot addr.
> -		 */
> -		smp_store_release(&slot->addr, NULL);
> -		/* Use the backup slot for context. */
> -		ctx->slot = backup_slot;
> +		hazptr_promote_to_backup_slot(item->ctx.ctx, slot);
>  	}
>  }
>  
> -- 
> 2.40.1
>