Re: [RFC PATCH v4 4/4] hazptr: Migrate per-CPU slots to backup slot on context switch

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-12-17 20:45, Mathieu Desnoyers wrote:
[...]
> +static inline
> +void hazptr_note_context_switch(void)
> +{
> +	struct hazptr_ctx *ctx;
> +
> +	list_for_each_entry(ctx, &current->hazptr_ctx_list, preempt_node) {
> +		struct hazptr_slot *slot;
> +
> +		if (hazptr_slot_is_backup(ctx, ctx->slot))
> +			continue;
> +		slot = hazptr_chain_backup_slot(ctx);
> +		/*
> +		 * Move hazard pointer from per-CPU slot to backup slot.
> +		 * This requires hazard pointer synchronize to iterate
> +		 * on per-CPU slots with load-acquire before iterating
> +		 * on the overflow list.
> +		 */
> +		WRITE_ONCE(slot->addr, ctx->slot->addr);
> +		/*
> +		 * store-release orders store to backup slot addr before
> +		 * store to per-CPU slot addr.
> +		 */
> +		smp_store_release(&ctx->slot->addr, NULL);

I missed this:

+               /* Use the backup slot for context. */
+               ctx->slot = slot;

This triggered the hazptr_release WARN_ON_ONCE() within modified
refscale tests.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com