Re: possible deadlock in lru_add_drain_all

[Byungchul Park]

On Wed, Nov 01, 2017 at 01:01:01PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 01, 2017 at 05:59:27PM +0900, Byungchul Park wrote:
> > On Tue, Oct 31, 2017 at 04:10:24PM +0100, Peter Zijlstra wrote:
> > > On Tue, Oct 31, 2017 at 03:58:04PM +0100, Michal Hocko wrote:
> > > > On Tue 31-10-17 15:52:47, Peter Zijlstra wrote:
> > > > [...]
> > > > > If we want to save those stacks; we have to save a stacktrace on _every_
> > > > > lock acquire, simply because we never know ahead of time if there will
> > > > > be a new link. Doing this is _expensive_.
> > > > > 
> > > > > Furthermore, the space into which we store stacktraces is limited;
> > > > > since memory allocators use locks we can't very well use dynamic memory
> > > > > for lockdep -- that would give recursive and robustness issues.
> > 
> > I agree with all you said.
> > 
> > But, I have a better idea, that is, to save only the caller's ip of each
> > acquisition as an additional information? Of course, it's not enough in
> > some cases, but it's cheep and better than doing nothing.
> > 
> > For example, when building A->B, let's save not only full stack of B,
> > but also caller's ip of A together, then use them on warning like:
> 
> Like said; I've never really had trouble finding where we take A. And

Me, either, since I know the way. But I've seen many guys who got
confused with it, which is why I suggested it.

But, leave it if you don't think so.

> for the most difficult cases, just the IP isn't too useful either.
> 
> So that would solve a non problem while leaving the real problem.