Re: [PATCH 0/4] notify userspace of offline->running transitions (v2)

[Mike Christie <michaelc@cs.wisc.edu>]

On 05/21/2012 11:49 AM, Kay Sievers wrote:
> On Mon, May 21, 2012 at 5:35 PM, Mike Christie <michaelc@cs.wisc.edu> wrote:
>> On 05/21/2012 06:50 AM, Hannes Reinecke wrote:
>>> On 05/18/2012 06:56 AM, michaelc@cs.wisc.edu wrote:
>>>> The following patches were made over the misc branch of the scsi tree.
>>>>
>>>> The patches fix a issue where if the device is offlined or IO is
>>>> failed due to fast_io_fail (fc) /recovery_tmo (iscsi) then comes
>>>> back, apps do not have a way a nice way to figure out the state
>>>> has transitioned to running. Apps have to either poll the sysfs state
>>>> file or send a SG IO to figure it out. With the patch apps can listen
>>>> for the KOBJ CHANGE event like some of them (at least udev does) do
>>>> already.
>>>>
>>>> v2:
>>>> - Rebased to misc.
>>>>
>>> In principle, yes.
>>>
>>
>> ccing Kay.
>>
>>> However, when doing this, we're now sending 'CHANGE' uevents from
>>> SCSI devices. With the potential of putting _quite_ some strain on udev.
>>> Kay explicitely debarred me from using uevents for my SCSI sense
>>
>> Kay told me to do it this way :) In this case udev was the app we
>> discovered the issue with, so maybe that is the diff.
> 
> Hah, I basically only told you not to use online/offline events. :)

Yeah, I guess you guys got me confused :) Harold said to send an event
for this issue. Then later you said "please always use change". I
thought you and Harold were in sync :(

> 
> Uevents are ok to use if we can be sure there is _never_ a storm of
> events. Uevents are not meant to handle large amounts of events
> happening at the same time. They must never be used to handle things
> like reporting errors which are not limited in their rate, or where
> many devices might send similar events at the same time in a row.
> 

In this case you can get many devices sending the same events at the
same time, because when the transport/connection comes back you all the
device accessed through that connection will be set to running at the
same time.

So what is the fix for this case? Just send one event for the host or
transport/connection then have udev loop over all devices accessed
through that object and fix things up? If so, what type of event do you
want? CHANGE event plus some other info to indicate to look at child
devices?