Re: [bisected] Re: todays git: WARNING: at drivers/ata/libata-sff.c:1017ata_sff_hsm_move+0x45e/0x750()

[Sergei Shtylyov]

Hello, I wrote:

> > >    All the S/G counts printed out were divisible by 4 (36 for 
> > > INQUIRY and 96 for REQUSET SENSE). It's the *actual* byte count for 
> > > the REQUEST SENSE that's no divisible. The SCSI/ATAPI devices are 
> > > free to sent less data than requested on non block transfer commands.
>
> > That is just fine - if the sg list is not corrupt or being mishandled 
> > and
> > the atapi pio code is not buggy.
>
> > RTFS a bit and it becomes obvious that the core libata code has a bug:
>
>    Oh, I have already... and saw where the issue could be. It just 
> wasn't obvious why 32-bit PIO triggered it.

 Got it now, however the issue doesn't seem as evident simple to me...

> > From libata-sff.c:
> >         /* consumed can be larger than count only for the last 
> > transfer */
> >         WARN_ON_ONCE(qc->cursg && count != consumed);
> >
> > The big clue turns out to be that the code doesn't match the comment.
> >
> > Next note the check on qc->cursg. If my input sg list is a 36 byte 
> > single
> > sg entry then qc->cursg should be NULL by the WARN_ON() - but it isn't.
>
>    I think it's still not NULL because qc->cursg_ofs == sg->length 
> check was *not* true right above, hence sg_next() wasn't called...
>
> > If qc->cursg is NULL when the sg_next() is run then we don't warn 
> > because
> > we are quite happy with the last segment being padded or underrunning.
>
>    I don't think that sg_next() is called on an underrun segment. And 
> here lies the mistake.
>
> > What we actually want to explode on is a case where we transfer more
> > bytes than are wanted and where there are more sg entries to perform 
> > - at
> > that point we would corrupt.
>
> > So at least one failure case is
>
> >     Core code issues an SG list for 96 bytes
> >     Drive indicates it wishes to return 18 bytes
>
> >     data_xfer transfers 18 bytes + 2 padding (correctly) -> 20 bytes

  Correctly indeed? I'm not at all sure it's correct to read an extra 
16-bit word off the device when it thinks it's already done with the 
data transfer. This is not the same as to read 16-bit word and ignore 
its MSB as it happened. The same concern about the writes... Note that 
the IDE code doesn't do this...

MBR, Sergei


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