Re: IDE Hotswap

From: Alan Cox (alan_at_lxorguk.ukuu.org.uk)
Date: 09/29/04

  • Next message: Andrea Arcangeli: "Re: heap-stack-gap for 2.6" 
    To: Bartlomiej Zolnierkiewicz <bzolnier@elka.pw.edu.pl>
Date:	Wed, 29 Sep 2004 15:35:17 +0100

    On Mer, 2004-09-29 at 13:08, Bartlomiej Zolnierkiewicz wrote:
    > > Even in 2.4 ide drive hotplug was easy. The drive hotplug comes out
    > > trivially because your controllers are fairly constant. As we all know
    > > driver level hotplug is a bit trickier although the block layer has
    > > really made this vastly easier in 2.6
    > >
    > > For drive level hotplug you don't actually need refcounting at all
    > > providing you've got a couple of locking issues dealt with.
    >
    > These issues can't be solved without refcounting.

    So you keep saying, but you refcount objects that are going away, you
    don't need to refcount objects that are staying put.

    > Feel free to probe me wrong, you can start with fixing
    > ->open vs unregister race (drive->usage involved). :)

    Doesn't occur in the 2.4 situation or the 2.6 stuff with the locking in
    the 2.6.8.1-ac patch.

    > > Firstly the drive never goes away as a high level object (in fact you
    > > don't want it to as then you can't ioctl it to make it come back!). That
    > > means the upper layers don't know anything about it.
    >
    > ioctls on not present devices are layering VIOLATION

    Oh dear then I guess most of Linux is misdesigned. You aren't thinking
    about the semantics at all.

    If /dev/hda is a CD-ROM drive I can issue commands to it with no CD
    present. Thats not a layering violation, and its how the IDE code works.
    So whats the difference between hotplugging a drive and removing
    a CD. Both are removing the media but leaving the controller behind.

    On that item I think you are talking out of your backside.

    > > At the IDE layer the 2.4 code simply enforced the rule that you must be
    > > the only opener of the device in order to hot unplug it. That means we
    >
    > "enforced" - there are a couple of races, sorry but ROTFL
    >
    > > know its quiescent and not mounted. The only 2.4 race I know about is
    >
    > - double unlock obvious mistake
    Details ?
    > - ->open() vs unregister
    unregister is hot plug controller not drive and thats unfixable in 2.4
    > - /proc races (the same you fixed in your 2.6 patch)
    yeah that lot postdates the 2.4 work. hotplug drives is not the cause
    however.
    > - ioctl races
    Details ?

    > gendisk layer and block layer enforces you to make /dev/hda disappear.

    No it does not. The block layer couldn't give a flying **** whether
    /dev/hda disappears or not. SCSI devices that are offlined don't need to
    disappear either you just hand back commands with an error. You know -
    like every CD-ROM does...

    > Does sysfs ring any bells? Ask viro about static objects vs sysfs.
    > And yes not only gendisk and block enforces this, Patrick added basic,
    > premature sysfs support to IDE driver in the middle of 2.5 series.
    >
    > We can get back to discussion when you get familiar with issues involved.

    Ditto...

    Alan

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