Re: [PATCH v3 1/6] block: add disk sequence number

[Hannes Reinecke]

On 6/23/21 4:07 PM, Luca Boccassi wrote:
> On Wed, 2021-06-23 at 16:01 +0200, Hannes Reinecke wrote:
> > On 6/23/21 3:51 PM, Lennart Poettering wrote:
> > > On Mi, 23.06.21 15:10, Matteo Croce (mcroce@xxxxxxxxxxxxxxxxxxx) wrote:
> > >
> > > > On Wed, Jun 23, 2021 at 1:49 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > > > > On Wed, Jun 23, 2021 at 12:58:53PM +0200, Matteo Croce wrote:
> > > > > > +void inc_diskseq(struct gendisk *disk)
> > > > > > +{
> > > > > > +     static atomic64_t diskseq;
> > > > >
> > > > > Please don't hide file scope variables in functions.
> > > >
> > > > I just didn't want to clobber that file namespace, as that is the only
> > > > point where it's used.
> > > >
> > > > > Can you explain a little more why we need a global sequence count vs
> > > > > a per-disk one here?
> > > >
> > > > The point of the whole series is to have an unique sequence number for
> > > > all the disks.
> > > > Events can arrive to the userspace delayed or out-of-order, so this
> > > > helps to correlate events to the disk.
> > > > It might seem strange, but there isn't a way to do this yet, so I come
> > > > up with a global, monotonically incrementing number.
> > >
> > > To extend on this and given an example why the *global* sequence number
> > > matters:
> > >
> > > Consider you plug in a USB storage key, and it gets named
> > > /dev/sda. You unplug it, the kernel structures for that device all
> > > disappear. Then you plug in a different USB storage key, and since
> > > it's the only one it will too be called /dev/sda.
> > >
> > > With the global sequence number we can still distinguish these two
> > > devices even though otherwise they can look pretty much identical. If
> > > we had per-device counters then this would fall flat because the
> > > counter would be flushed out when the device disappears and when a device
> > > reappears under the same generic name we couldn't assign it a
> > > different sequence number than before.
> > >
> > > Thus: a global instead of local sequence number counter is absolutely
> > > *key* for the problem this is supposed to solve
> > Well ... except that you'll need to keep track of the numbers (otherwise
> > you wouldn't know if the numbers changed, right?).
> > And if you keep track of the numbers you probably will have to implement
> > an uevent listener to get the events in time.
> > But if you have an uevent listener you will also get the add/remove
> > events for these devices.
> > And if you get add and remove events you can as well implement sequence
> > numbers in your application, seeing that you have all information
> > allowing you to do so.
> > So why burden the kernel with it?
> >
> > Cheers,
> >
> > Hannes
>
> Hi,
>
> We need this so that we can reliably correlate events to instances of a
> device. Events alone cannot solve this problem, because events _are_
> the problem.
In which sense?
Yes, events can be delayed (if you list to uevents), but if you listen 
to kernel events there shouldn't be a delay, right?

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
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