Re: [RFC PATCH] scsi: libsas: fix WARN on device removal

[John Garry]

On 09/11/2016 20:35, Dan Williams wrote:
> On Wed, Nov 9, 2016 at 11:09 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> > On Wed, Nov 9, 2016 at 9:36 AM, John Garry <john.garry@xxxxxxxxxx> wrote:
> > > On 09/11/2016 12:28, John Garry wrote:
> > > > On 03/11/2016 14:58, John Garry wrote:
> > > > > The following patch introduces an annoying WARN
> > > > > when a device is removed from the SAS topology:
> > > > > [SCSI] libsas: prevent domain rediscovery competing with ata error
> > > > > handling
> > > >
> > > > Are there any views on this patch? I would have thought that the parties
> > > > who use the drivers based on libsas would be interested in fixing this
> > > > bug.
> > >
> > > I should have added the before and after logs earlier, so the issue is
> > > illustrated. Now attached. When a 24-port expander is unplugged we get >6k
> > > lines of WARN on the console, lasting >30 seconds. Not nice.
> >
> > I might be mistaken, but this patch seems functionally identical to
> > this attempt:
> >
> > http://marc.info/?l=linux-scsi&m=143459794823595&w=2

Hi Dan,

They're not the same. I don't see how your solution properly deals with 
remote sas_port deletion.

When we unplug a device connected to an expander, can't the sas_port be 
deleted twice, in sas_unregister_devs_sas_addr() from domain 
revalidation and also now in sas_destruct_devices()? I think that this 
gives a NULL dereference.
And we still get the WARN as the sas_port has still been deleted before 
the device.

In my solution, we should always delete the sas_port after the attached 
device.

> > i.e. it moves the port destruction to the workqueue and still suffers
> > from the flutter problem:
> >
> > http://marc.info/?l=linux-scsi&m=143801026028006&w=2
> > http://marc.info/?l=linux-scsi&m=143801971131073&w=2
> >
> > Perhaps we instead need to quiet this warning?
> >
> > http://marc.info/?l=linux-scsi&m=143802229932175&w=2

I have not seen the flutter issue. I am just trying to solve the 
horrible WARN dump.
However I do understand that there may be a issue related to how we 
queue the events; there was a recent attempt to fix this, but it came to 
nothing:
https://www.spinics.net/lists/linux-scsi/msg99991.html

Cheers,
John

> Alternatively we need a mechanism to cancel in-flight port shutdown
> requests when we start re-attaching devices before queued port
> destruction events have run.
>
> .