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

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Message-ID: <9ea470500906020608o14f8bc8dwaedabff70c1cb707@mail.gmail.com>
Date:	Tue, 2 Jun 2009 15:08:27 +0200
From:	Borislav Petkov <petkovbb@...glemail.com>
To:	Bartlomiej Zolnierkiewicz <bzolnier@...il.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

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.

-- 
Regards/Gruss,
Boris
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