Re: [PATCH take2 3/5] dio: formalize bio counters as a dio reference count

[Andrew Morton <akpm@osdl.org>]

On Mon,  2 Oct 2006 16:21:35 -0700 (PDT)
Zach Brown <zach.brown@oracle.com> wrote:

> dio: formalize bio counters as a dio reference count
> 
> Previously we had two confusing counts of bio progress.  'bio_count' was
> decremented as bios were processed and freed by the dio core.  It was used to
> indicate final completion of the dio operation.  'bios_in_flight' reflected how
> many bios were between submit_bio() and bio->end_io.  It was used by the sync
> path to decide when to wake up and finish completing bios and was ignored
> by the async path.
> 
> This patch collapses the two notions into one notion of a dio reference count.
> bios hold a dio reference when they're between submit_bio and bio->end_io.
> 
> Since bios_in_flight was only used in the sync path it is now equivalent
> to dio->refcount - 1 which accounts for direct_io_worker() holding a 
> reference for the duration of the operation.
> 
> dio_bio_complete() -> finished_one_bio() was called from the sync path after
> finding bios on the list that the bio->end_io function had deposited.
> finished_one_bio() can not drop the dio reference on behalf of these bios now
> because bio->end_io already has.  The is_async test in finished_one_bio() meant
> that it never actually did anything other than drop the bio_count for sync
> callers.  So we remove its refcount decrement, don't call it from
> dio_bio_complete(), and hoist its call up into the async dio_bio_complete()
> caller after an explicit refcount decrement.  It is renamed dio_complete_aio()
> to reflect the remaining work it actually does.
> 
> ...
>  
> +static int wait_for_more_bios(struct dio *dio)
> +{
> +	assert_spin_locked(&dio->bio_lock);
> +
> +	return (atomic_read(&dio->refcount) > 1) && (dio->bio_list == NULL);
> +}

This function isn't well-named.

> @@ -1103,7 +1088,11 @@ direct_io_worker(int rw, struct kiocb *i
>  		}
>  		if (ret == 0)
>  			ret = dio->result;
> -		finished_one_bio(dio);		/* This can free the dio */
> +
> +		/* this can free the dio */
> +		if (atomic_dec_and_test(&dio->refcount))
> +			dio_complete_aio(dio);

So...  how come it's legitimate to touch *dio if it can be freed by now? 
(iirc, it's legit, but a comment explaining this oddity is needed).