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SubjectRe: [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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