linux-kernel - Re: [PATCH v8 06/14] lockdep: Detect and handle hist

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Date:   Mon, 14 Aug 2017 15:05:22 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Byungchul Park <max.byungchul.park@...il.com>
Cc:     Byungchul Park <byungchul.park@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, tglx@...utronix.de,
        Michel Lespinasse <walken@...gle.com>, kirill@...temov.name,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, akpm@...ux-foundation.org, willy@...radead.org,
        npiggin@...il.com, kernel-team@....com
Subject: Re: [PATCH v8 06/14] lockdep: Detect and handle hist_lock ring
 buffer overwrite

On Fri, Aug 11, 2017 at 10:06:37PM +0900, Byungchul Park wrote:
> On Fri, Aug 11, 2017 at 6:44 PM, Byungchul Park <byungchul.park@....com> wrote:
> > On Fri, Aug 11, 2017 at 05:52:02PM +0900, Byungchul Park wrote:
> >> On Fri, Aug 11, 2017 at 04:03:29PM +0800, Boqun Feng wrote:
> >> > Thanks for taking a look at it ;-)
> >>
> >> I rather appriciate it.
> >>
> >> > > > @@ -5005,7 +5003,7 @@ static int commit_xhlock(struct cross_lock *xlock, struct hist_lock *xhlock)
> >> > > >  static void commit_xhlocks(struct cross_lock *xlock)
> >> > > >  {
> >> > > >         unsigned int cur = current->xhlock_idx;
> >> > > > -       unsigned int prev_hist_id = xhlock(cur).hist_id;
> >> > > > +       unsigned int prev_hist_id = cur + 1;
> >> > >
> >> > > I should have named it another. Could you suggest a better one?
> >> > >
> >> >
> >> > I think "prev" is fine, because I thought the "previous" means the
> >> > xhlock item we visit _previously_.
> >> >
> >> > > >         unsigned int i;
> >> > > >
> >> > > >         if (!graph_lock())
> >> > > > @@ -5030,7 +5028,7 @@ static void commit_xhlocks(struct cross_lock *xlock)
> >> > > >                          * hist_id than the following one, which is impossible
> >> > > >                          * otherwise.
> >> > >
> >> > > Or we need to modify the comment so that the word 'prev' does not make
> >> > > readers confused. It was my mistake.
> >> > >
> >> >
> >> > I think the comment needs some help, but before you do it, could you
> >> > have another look at what Peter proposed previously? Note you have a
> >> > same_context_xhlock() check in the commit_xhlocks(), so the your
> >> > previous overwrite case actually could be detected, I think.
> >>
> >> What is the previous overwrite case?
> >>
> >> ppppppppppwwwwwwwwwwwwiiiiiiiii
> >> iiiiiiiiiiiiiii................
> >>
> >> Do you mean this one? I missed the check of same_context_xhlock(). Yes,
> >> peterz's suggestion also seems to work.
> >>
> >> > However, one thing may not be detected is this case:
> >> >
> >> >             ppppppppppppppppppppppppppppppppppwwwwwwww
> >> > wrapped >   wwwwwww
> >>
> >> To be honest, I think your suggestion is more natual, with which this
> >> case would be also covered.
> >>
> >> >
> >> >     where p: process and w: worker.
> >> >
> >> > , because p and w are in the same task_irq_context(). I discussed this
> >> > with Peter yesterday, and he has a good idea: unconditionally do a reset
> >> > on the ring buffer whenever we do a crossrelease_hist_end(XHLOCK_PROC).
> >
> > Ah, ok. You meant 'whenever _process_ context exit'.
> >
> > I need more time to be sure, but anyway for now it seems to work with
> > giving up some chances for remaining xhlocks.
> >
> > But, I am not sure if it's still true even in future and the code can be
> > maintained easily. I think your approach is natural and neat enough for
> > that purpose. What problem exists with yours?
> 

My approach works but it has bigger memmory footprint than Peter's, so I
asked about whether you could consider Peter's approach.

> Let me list up the possible approaches:
> 
> 0. Byungchul's approach

Your approach requires(additionally):

	MAX_XHLOCKS_NR * sizeof(unsigned int) // because of the hist_id field in hist_lock
	+ 
	(XHLOCK_CXT_NR + 1) * sizeof(unsigned int) // because of fields in task_struct

bytes per task.

> 1. Boqun's approach

My approach requires(additionally):

	MAX_XHLOCKS_NR * sizeof(unsigned int) // because of the hist_id field in hist_lock

bytes per task.

> 2. Peterz's approach

And Peter's approach requires(additionally):

	1 * sizeof(unsigned int)

bytes per task.

So basically we need some tradeoff between memory footprints and history
precision here.

> 3. Reset on process exit
> 
> I like Boqun's approach most but, _whatever_. It's ok if it solves the problem.
> The last one is not bad when it is used for syscall exit, but we have to give
> up valid dependencies unnecessarily in other cases. And I think Peterz's
> approach should be modified a bit to make it work neatly, like:
> 
> crossrelease_hist_end(...)
> {
> ...
>        invalidate_xhlock(&xhlock(cur->xhlock_idx_max));
> 
>        for (c = 0; c < XHLOCK_CXT_NR; c++)
>               if ((cur->xhlock_idx_max - cur->xhlock_idx_hist[c]) >=
> MAX_XHLOCKS_NR)
>                      invalidate_xhlock(&xhlock(cur->xhlock_idx_hist[c]));
> ...
> }
> 

Haven't looked into this deeply, but my gut feeling is this is
unnecessary, will have a deep look.

Regards,
Boqun

> And then Peterz's approach can also work, I think.
> 
> ---
> Thanks,
> Byungchul

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