linux-kernel - Re: [PATCH 1/2] ide-tape: change IDE_AFLAG_IGNORE

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Message-Id: <200906021517.53027.bzolnier@gmail.com>
Date:	Tue, 2 Jun 2009 15:17:52 +0200
From:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
To:	Borislav Petkov <petkovbb@...glemail.com>
Cc:	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] ide-tape: change IDE_AFLAG_IGNORE_DSC non-atomically

On Tuesday 02 June 2009 15:08:27 Borislav Petkov wrote:
> Hi,
> 
> On Tue, Jun 2, 2009 at 12:18 PM, Bartlomiej Zolnierkiewicz
> <bzolnier@...il.com> wrote:
> 
> ..
> 
> >> --- a/drivers/ide/ide-tape.c
> >> +++ b/drivers/ide/ide-tape.c
> >> @@ -656,15 +656,24 @@ static ide_startstop_t idetape_do_request(ide_drive_t *drive,
> >>
> >>       if ((drive->dev_flags & IDE_DFLAG_DSC_OVERLAP) == 0 &&
> >>           (rq->cmd[13] & REQ_IDETAPE_PC2) == 0)
> >> -             set_bit(IDE_AFLAG_IGNORE_DSC, &drive->atapi_flags);
> >> +             drive->atapi_flags |= IDE_AFLAG_IGNORE_DSC;
> >>
> >>       if (drive->dev_flags & IDE_DFLAG_POST_RESET) {
> >> -             set_bit(IDE_AFLAG_IGNORE_DSC, &drive->atapi_flags);
> >> +             drive->atapi_flags |= IDE_AFLAG_IGNORE_DSC;
> >>               drive->dev_flags &= ~IDE_DFLAG_POST_RESET;
> >>       }
> >>
> >> -     if (!test_and_clear_bit(IDE_AFLAG_IGNORE_DSC, &drive->atapi_flags) &&
> >> -         (stat & ATA_DSC) == 0) {
> >> +     /*
> >> +      * This is a precaution for IDE_AFLAG_IGNORE_DSC being conditionally set
> >> +      * above. We don't need a stronger enforcement of ordering because the
> >> +      * read below cannot precede the earlier write out-of-order since it is
> >> +      * to the same location. Also, since we have the ide port locked during
> >> +      * the ->do_request(), we only have to be aware of gcc reordering stuff.
> >> +      */
> >> +     barrier();
> >
> > Are you seeing a real problem with gcc here?  No sane compiler should need
> > a barrier() here (we would probably need zillions of them in kernel if it
> > really does).
> 
> No, this is just a precaution. The asm I checked looked fine but since
> the flag is set and right afterwards checked, it will be bad if this
> somehow got reordered. I actually haven't checked whether anything like
> that would be possible, at all.

Please remove it then.
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