Messages in this thread | | | Subject | Re: IDE Hotswap | From | Alan Cox <> | Date | Wed, 29 Sep 2004 15:35:17 +0100 |
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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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