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

[Boqun Feng]

On Sat, Sep 19, 2026 at 12:42:48PM +0100, Boqun Feng wrote:
> On Fri, Sep 18, 2026 at 05:00:47PM -0700, Paul E. McKenney wrote:
> > From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Boqun Feng <boqun@xxxxxxxxxx>
> > Cc: <rcu@xxxxxxxxxxxxxxx>
> > Cc: <lkmm@xxxxxxxxxxxxxxx>
> > ---
> >  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?
> 

Obivioulsy I'm missing patch #24, never mind then :)

Regards,
Boqun

> /*
>  * 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
> >