Re: [PATCH v7 06/16] lockdep: Detect and handle hist_lock ring buffer overwrite

From: Peter Zijlstra <peterz@infradead.org>
To: Byungchul Park <byungchul.park@lge.com>
Cc: Byungchul Park <max.byungchul.park@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	tglx@linutronix.de, Michel Lespinasse <walken@google.com>,
	boqun.feng@gmail.com, kirill@shutemov.name,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, akpm@linux-foundation.org,
	willy@infradead.org, npiggin@gmail.com, kernel-team@lge.com
Subject: Re: [PATCH v7 06/16] lockdep: Detect and handle hist_lock ring buffer overwrite
Date: Fri, 21 Jul 2017 15:54:20 +0200	[thread overview]
Message-ID: <20170721135420.gadjqv6hian4yzgq@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170714064210.GK20323@X58A-UD3R>

On Fri, Jul 14, 2017 at 03:42:10PM +0900, Byungchul Park wrote:
> On Thu, Jul 13, 2017 at 08:23:33PM +0900, Byungchul Park wrote:
> > On Thu, Jul 13, 2017 at 8:12 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > > On Thu, Jul 13, 2017 at 12:29:05PM +0200, Peter Zijlstra wrote:
> > >> On Thu, Jul 13, 2017 at 07:09:53PM +0900, Byungchul Park wrote:
> > >> > On Thu, Jul 13, 2017 at 11:50:52AM +0200, Peter Zijlstra wrote:
> > >> > >   wait_for_completion(&C);
> > >> > >     atomic_inc_return();
> > >> > >
> > >> > >                                   mutex_lock(A1);
> > >> > >                                   mutex_unlock(A1);
> > >> > >
> > >> > >
> > >> > >                                   <IRQ>
> > >> > >                                     spin_lock(B1);
> > >> > >                                     spin_unlock(B1);
> > >> > >
> > >> > >                                     ...
> > >> > >
> > >> > >                                     spin_lock(B64);
> > >> > >                                     spin_unlock(B64);
> > >> > >                                   </IRQ>
> > >> > >
> > >> > >
> > >
> > > Also consider the alternative:
> > >
> > >                                         <IRQ>
> > >                                           spin_lock(D);
> > >                                           spin_unlock(D);
> > >
> > >                                           complete(&C);
> > >                                         </IRQ>
> > >
> > > in which case the context test will also not work.
> > 
> > Context tests are done on xhlock with the release context, _not_
> > acquisition context. For example, spin_lock(D) and complete(&C) are
> > in the same context, so the test would pass in this example.

The point was, this example will also link C to B*.

(/me copy paste from older email)

That gives:

        xhist[ 0] = A1
        xhist[ 1] = B1
        ...
        xhist[63] = B63

then we wrap and have:

        xhist[0] = B64

then we rewind to 1 and invalidate to arrive at:

        xhist[ 0] = B64
        xhist[ 1] = NULL   <-- idx
        xhist[ 2] = B2
        ...
        xhist[63] = B63

Then we do D and get

        xhist[ 0] = B64
        xhist[ 1] = D   <-- idx
        xhist[ 2] = B2
        ...
        xhist[63] = B63

And now there is nothing that will invalidate B*, after all, the
gen_id's are all after C's stamp, and the same_context_xhlock() test
will also pass because they're all from IRQ context (albeit not the
same, but it cannot tell).

Does this explain? Or am I still missing something?

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