Re: [RFC 1/4] lockdep: additional lock specific information when dumping locks

[Peter Zijlstra]

On Mon, Jan 12, 2015 at 11:06:17AM -0500, Sasha Levin wrote:
> On 01/12/2015 10:37 AM, Peter Zijlstra wrote:
> > On Mon, Jan 12, 2015 at 10:12:38AM -0500, Sasha Levin wrote:
> >> The reason for my patch is simple: 
> > 
> > That might have maybe been good changelog material?
> > 
> >> I'm fuzzing with hundreds of worker threads
> >> which at some point trigger a complete system lockup for some reason.
> >>
> >> When lockdep dumps the list of held locks it shows that pretty much every one
> >> of those threads is holding the lock which caused the lockup, which is incorrect
> >> because it considers locks in the process of getting acquired as "held".
> >>
> >> This is my solution to that issue. I wanted to know which one of the threads is
> >> really holding the lock rather than just waiting on it.
> >>
> >> Is there a better way to solve that problem?
> > 
> > Sure, think moar, if the accompanying stack trace is in the middle
> > of the blocking primitive, ignore the top held lock ;-)
> 
> Tried that, it's a pain.
> 
> Consider this scenario:
> 
> Process A	|	Process B	| Process C-[...]
> ----------------|-----------------------|----------------
> mutex_lock(x)	|			|
> [busy working]	|			|
> 		|	mutex_lock(z)	|
> 		|	mutex_lock(x)	|
> 		|	[waiting on x]	|
> 		|			|	mutex_lock(z)
> 		|			|	[waiting on z]
> 
> So at the end of all of that I have 1000 processes waiting on 'z', while
> the process that has 'z' is waiting on 'x'. So if I look at which processes
> are not stuck inside a blocking primitive I'll miss on process B., and it's
> link between process A and process B.

I never said to ignore everything for tasks blocked inside locking
primitives, only ignore the top held.

But sure, I can relate how large numbers make this painful.

> > Alternatively, make better/more use of lock_acquired() and track the
> > acquire vs acquired information in the held_lock (1 bit) and look at it
> > when printing.
> 
> We could do that, but then we'd lose the ability to get information out of
> locks, what's the benefit of doing that?

That's mission creep; you never stated that as a goal.

One of the reasons i'm not particularly keen on it is because it creates
a circular dependency between lock implementations and lockdep. It also
creates asymmetry between lock types/capabilty.
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