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

[Bartlomiej Zolnierkiewicz]

On 11/23/05, Tejun Heo <htejun@xxxxxxxxx> 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).

Bartlomiej
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