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

[Zach Brown <zach.brown@oracle.com>]

>> +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.

Sure, I'll try harder.

> 
>> @@ -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? 

Indeed!  This point in the patchset is retaining the existing behaviour
where the 'should_wait' direct_io_worker() path and the
finished_one_bio() path very carefully keep their tests in sync so that
only one of them ends up freeing the dio.

This particular patch doesn't change this part of the behaviour, it's
just bringing the dec and complete that was previously hidden in
finished_one_bio() up into its caller.  So I kept the comment.

This mess is replaced with a proper

	if (dec_and_test(dio->refcount))
		kfree(dio)

construct in the final patch.  I hope that means we don't need to patch
in some comments that we later remove.

- z