linux-kernel - Re: [PATCH 1/2] libata: get rid of ATA_MAX_QUEUE loop in ata_qc_complete

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Date:	Thu, 23 Oct 2008 15:40:18 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	jeff@...zik.org
Subject: Re: [PATCH 1/2] libata: get rid of ATA_MAX_QUEUE loop in ata_qc_complete_multiple()

On Thu, Oct 23 2008, Jens Axboe wrote:
> On Thu, Oct 23 2008, Tejun Heo wrote:
> > while (done_mask) {
> >         struct ata_queued_cmd *qc;
> >         unsigned int next = __ffs(done_mask);
> > 
> >         tag += next;
> >         if ((qc = ata_qc_from_tag(ap, tag))) {
> >                 ata_qc_complete(qc);
> >                 nr_done++;
> >         }
> >         next++;
> >         tag += next;
> >         done_mask >>= next;
> > }
> 
> That doesn't work (you're adding next to tag twice), it needs a little
> tweak:
> 
> while (done_mask) {
>         struct ata_queued_cmd *qc;
>         unsigned int next = __ffs(done_mask);
> 
>         if ((qc = ata_qc_from_tag(ap, tag + next))) {
>                 ata_qc_complete(qc);
>                 nr_done++;
>         }
>         next++;
>         tag += next;
>         done_mask >>= next;
> }
> 
> and I think it should work. Not tested yet :-)

Pondered some more, and it can't work. The problem is that if we
complete tag 31, we attempt to shift done_mask down by 32 bits. On a
32-bit arch, that's not defined. So we DO need a check like the existing
one, or something similar.

So I don't think we need to make changes to this patch either, at least
unless one of you can come up with a better check that avoids a branch.

-- 
Jens Axboe

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