On Fri, Sep 18, 2026 at 05:00:47PM -0700, Paul E. McKenney wrote:
> From: Mathieu Desnoyers <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Paul E. McKenney <[email protected]>
> Cc: Boqun Feng <[email protected]>
> Cc: <[email protected]>
> Cc: <[email protected]>
> ---
> 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
>
