Re: [PATCH linux-2.6-block:post-2.6.15 08/10] blk: update IDE to use new blk_ordered

[Tejun <htejun@gmail.com>]

Jens Axboe wrote:
> On Wed, Nov 23 2005, Bartlomiej Zolnierkiewicz wrote:
> 
>>On 11/23/05, Tejun Heo <htejun@gmail.com> wrote:
>>
>>>On Tue, Nov 22, 2005 at 09:36:09AM +0100, Bartlomiej Zolnierkiewicz wrote:
>>>[--snip--]
>>>
>>>>>Ordered requests are processed in the following order.
>>>>>
>>>>>1. barrier bio reaches blk queue
>>>>>
>>>>>2. barrier req queued in order
>>>>>
>>>>>3. when barrier req reaches the head of the request queue, it gets
>>>>>   interpreted into preflush-barrier-postflush requests sequence
>>>>>   and queued.  ->prepare_flush_fn is called in this step.
>>>>>
>>>>>4. When all three requests complete, the ordered sequence ends.
>>>>>
>>>>>Adding !drive->wcache test to idedisk_prepare_flush, which in turn
>>>>>requires adding ->prepare_flush_fn error handling to blk ordered
>>>>>handling, prevents flushes for barrier requests between step#1 and
>>>>
>>>>Why for !drive->wcache flush can't be consider as successful
>>>>like it was before these changes...
>>>>
>>>>
>>>>>step#3.  We can still have flush reqeuests between #3 and #4 after
>>>>>wcache is turned off.
>>>>
>>>>ditto
>>>>
>>>
>>>I think we have two alternatives here - both have some problems.
>>>
>>>1. make ->prepare_flush_fn return some code to tell blk layer skip
>>>   the flush as the original code did.
>>>
>>>This is what you're proposing, I guess.  The reason why I'm reluctant
>>>to take this approach is that there still remains window of error
>>>between #3 and #4.  The flush requests could already be prepared and
>>>in the queue when ->wcache is turned off.  AFAICS, the original code
>>>had the same problem, although the window was narrower.
>>>
>>>2. complete flush commands successfully in execute_drive_cmd() if wcache
>>>   is not enabled.
>>>
>>>This approach fixes all cases but the implementation would be a bit
>>>hackish.  execute_drive_cmd() will have to look at the command and
>>>ide_disk->wcache to determine if a special command should be completed
>>>successfully without actually executing it.  Maybe we can add a
>>>per-HL-driver callback for that.
>>>
>>>Bartlomiej, I'm not really sure about both of above approaches.  My
>>>humble opinion is that benefits brought by both of above aren't worth
>>>the added complexity.  The worst can happen is a few IDE command
>>>failures caused by executing flush command on a wcache disabled drive.
>>>And that would happen only when the user turns off wcache while
>>>barrier sequence is *in progress*.
>>>
>>>Hmmm... What do you think?  It's your call.  I'll do what you choose.
>>
>>Hmm... both solutions sucks.  After second thought I agree with you
>>w.r.t to original changes (just remember to document them in the patch
>>description).
> 

Yeap, both suck.  Glad to hear that.  :-)

> 
> Me too. Plus on most drives a flush cache command on a drive with write
> back caching disabled will succeed, not fail. t13 is about to mandate
> that behaviour as well, and honestly I think this is the logical way to
> implement it in a drive (the flush just becomes a noop).
> 
> So Tejun, where do we stand with this patch series? Any changes over
> what you posted last week? I'd like to get this merged in the block
> tree, get it in -mm and merged for 2.6.16 if things work out.
> 

Hi, Bartlomiej.  Hi, Jens.

Currently, there are only two more things to do before getting this 
thing into the -mm tree.

1. Adjusting this patch (update-ide) after merging Bartlomiej's 
del_gendisk() fix patch.

2. Update ide-fua patch as discussed and get it reviewed by Bartlomiej.

Other than above two, I think we're ready.  I'll post update ide-fua 
patch later today.

Thanks.

-- 
tejun