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Date: Thu, 13 Jul 2017 19:09:53 +0900
From: Byungchul Park <byungchul.park@lge.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, tglx@linutronix.de, walken@google.com,
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Subject: Re: [PATCH v7 06/16] lockdep: Detect and handle hist_lock ring
 buffer overwrite
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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

> 	xhist[ 2] = B2
> 	...
> 	xhist[63] = B63
> 
> 
> Then we do A2 and get
> 
> 	xhist[ 0] = B64
> 	xhist[ 1] = A2   <-- idx
> 	xhist[ 2] = B2
> 	...
> 	xhist[63] = B63

So invalidating only one is enough.