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Date: Wed, 1 Nov 2017 13:01:01 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Byungchul Park <byungchul.park@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>, Dmitry Vyukov <dvyukov@google.com>,
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Subject: Re: possible deadlock in lru_add_drain_all
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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
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.