Re: [PATCH RFC 10/20] ctlv: Prepare event queue support

[Jiri Pirko]

Tue, Oct 27, 2026 at 01:38:21PM +0100, yeoreum.yun@xxxxxxx wrote:
>Hi Jiri,
>
>
>[...]
>
>> +int ctlv_event_emit(struct ctlv_op_ctx *ctx)
>> +{
>> +	struct ctlv_device *ctlvdev = ctx->ctlvdev;
>> +	struct ctlv_event_record *rec, *next;
>> +	struct ctlv_file *cfile;
>> +	u32 len, wanted = 0;
>> +	LIST_HEAD(spare);
>> +	int ret;
>> +
>> +	if (WARN_ON_ONCE(ctx->file))
>> +		return -EINVAL;
>> +	ret = ctlv_event_finish(ctx);
>> +	if (ret)
>> +		goto out_free;
>> +
>> +	len = sizeof(struct ctlv_event_hdr) + CTLV_ALIGN(ctx->output.root->len);
>> +	scoped_guard(spinlock, &ctlvdev->events_lock) {
>> +		if (ctlvdev->dead_announced)
>> +			goto out_free;
>> +		list_for_each_entry(cfile, &ctlvdev->files, node)
>> +			if (test_bit(ctx->op->event_index, cfile->subscribed))
>> +				wanted++;
>> +	}
>
>Suppose at the time of here, the number of subscriber is 1.
>However, before calling the ctlv_event_deliver(), another task hold the
>event_lock() to register new subscriber. for example:
>
>
>Task 1 (emit)					Task 2 (new subscriber B)
>
>wanted = 1
>unlock events_lock.
>						Register subscription.
>allocate one record
>ctlv_event_delver()
>
>  grab events_lock.
>    Deliver former subscribe event to old one.
>    new subscriber B will call ctlv_event_queue_event() with NULL.
>
>When this happens, new subscriber B would receive overflow event
>via ctlv_event_queue_dropped() though it was registered right before.
>
>Is this intended? It doesn't make sense that subscribe B receives
>the overflow event in this situation...

Not intended. Will fix.

>
>
>> +	if (wanted) {
>> +		ctlv_event_copies(ctx, len, &spare, wanted);
>> +		ctlv_event_deliver(ctx, &spare);
>> +		list_for_each_entry_safe(rec, next, &spare, node)
>> +			kfree(rec);
>> +	}
>> +
>> +out_free:
>> +	ctlv_event_free(ctx);
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_NS_GPL(ctlv_event_emit, "CTLV");
>> +
>
>[...]
>
>Thanks!
>
>-- 
>Sincerely,
>Yeoreum Yun