Re: [PATCH 2/2] tracing/events: block: dev_t via driver core for plug and unplug events

[Omar Sandoval <osandov@osandov.com>]

On Mon, Apr 16, 2018 at 06:33:27PM +0200, Steffen Maier wrote:
> Hi Greg,
> 
> On 04/15/2018 10:31 AM, Greg Kroah-Hartman wrote:
> > On Fri, Apr 13, 2018 at 03:07:18PM +0200, Steffen Maier wrote:
> > > diff --git a/include/trace/events/block.h b/include/trace/events/block.h
> > > index a13613d27cee..cffedc26e8a3 100644
> > > --- a/include/trace/events/block.h
> > > +++ b/include/trace/events/block.h
> > > @@ -460,14 +460,18 @@ TRACE_EVENT(block_plug,
> > >   	TP_ARGS(q),
> > >   	TP_STRUCT__entry(
> > > +		__field( dev_t,		dev			)
> > >   		__array( char,		comm,	TASK_COMM_LEN	)
> > >   	),
> > >   	TP_fast_assign(
> > > +		__entry->dev = q->kobj.parent ?
> > > +		container_of(q->kobj.parent, struct device, kobj)->devt : 0;
> > 
> > That really really really scares me.  It feels very fragile and messing
> > with parent pointers is ripe for things breaking in the future in odd
> > and unexplainable ways.
> > 
> > And how can the parent be NULL?
> 
> I'm hoping for help by block layer experts.
> 
> I suppose block device unplug/removal could be a case. But I don't know the
> details how this works and if object release is protected while I/O is
> pending and new I/O is rejected beforehand. That might make my approach safe
> as it would not call the trace functions while the parent pointer changes.

We quiesce and freeze the queue before tearing it down, so it won't be
NULL while we're still doing I/O. Cc'ing Bart in case I'm lying to you,
though ;)

> > I don't know the block layer but this feels very wrong to me.  Are you
> > sure there isn't some other way to get this info?
> 
> No, I'm not sure at all. But I'm no block layer expert either. This is just
> an idea I had which did work for my cases and I'm looking for confirmation
> or denial by the experts. So if it's not safe from a block layer point of
> view either, then I have to ditch it.

There's not a pretty way to do this, but using the proper helpers would
be preferred:

disk_devt(dev_to_disk(kobj_to_dev(q->kobj.parent)))

The parent of a request_queue is always a gendisk, so this should always
work.

> -- 
> Mit freundlichen Grüßen / Kind regards
> Steffen Maier
> 
> Linux on z Systems Development
> 
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