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From: Byungchul Park <max.byungchul.park@gmail.com>
Date: Thu, 13 Jul 2017 20:19:14 +0900
Message-ID: <CANrsvRNAeL=Xdrbg5k56o5pC_p05LN7=rSrXLTHBJiXXFXk5Mw@mail.gmail.com>
Subject: Re: [PATCH v7 06/16] lockdep: Detect and handle hist_lock ring buffer overwrite
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Byungchul Park <byungchul.park@lge.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

On Thu, Jul 13, 2017 at 7:29 PM, Peter Zijlstra <peterz@infradead.org> 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>
>> >
>> >
>> >                                     mutex_lock(A2);
>> >                                     mutex_unlock(A2);
>> >
>> >                                     complete(&C);
>> >
>> >
>> > That gives:
>> >
>> >     xhist[ 0] = A1
>>
>> We have to rollback here later on irq_exit.
>>
>> The followings are ones for irq context.
>>
>> >     xhist[ 1] = B1
>> >     ...
>> >     xhist[63] = B63
>> >
>> > then we wrap and have:
>> >
>> >     xhist[0] = B64
>> >
>> > then we rewind to 1 and invalidate to arrive at:
>> >
>>
>> Now, whether xhist[0] has been overwritten or not is important. If yes,
>> xhist[0] should be NULL, _not_ xhist[1], which is one for irq context so
>> not interest at all.
>>
>> >     xhist[ 0] = B64
>> >     xhist[ 1] = NULL   <-- idx
>>
>> Therefore, it should be,
>>
>>       xhist[ 0] = NULL <- invalidate, cannot use it any more
>>       --- <- on returning back from irq context, start from here
>>       xhist[ 1] = B1 <-- obsolete history of irq
>
> Ah, so you rely on the same_context_xhlock() ? That doesn't work for
> hist (formerly work).

As I mentioned in cover-letter, I applied the rollback mechanism into work
(of workqueue) as well. So it works even for hist.

-- 
Thanks,
Byungchul

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