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Date: Thu, 13 Jul 2017 12:29:05 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Byungchul Park <byungchul.park@lge.com>
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 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).