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

[Peter Zijlstra]

On Thu, Jul 13, 2017 at 05:57:46PM +0900, Byungchul Park wrote:
> On Thu, Jul 13, 2017 at 10:14:42AM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 13, 2017 at 11:07:45AM +0900, Byungchul Park wrote:
> > > Does my approach have problems, rewinding to 'original idx' on exit and
> > > deciding whether overwrite or not? I think, this way, no need to do the
> > > drastic work. Or.. does my one get more overhead in usual case?
> > 
> > So I think that invalidating just the one entry doesn't work; the moment
> 
> I think invalidating just the one is enough. After rewinding, the entry
> will be invalidated and the ring buffer starts to be filled forward from
> the point with valid ones. When commit, it will proceed backward with
> valid ones until meeting the invalidated entry and stop.
> 
> IOW, in case of (overwritten)
> 
>          rewind to here
>          |
> ppppppppppiiiiiiiiiiiiiiii
> iiiiiiiiiiiiiii
> 
>          invalidate it on exit_irq
>          and start to fill from here again
>          |
> pppppppppxiiiiiiiiiiiiiiii
> iiiiiiiiiiiiiii
> 
>                     when commit occurs here
>                     |
> pppppppppxpppppppppppiiiii
> 
>          do commit within this range
>          |<---------|
> pppppppppxpppppppppppiiiii
> 
> So I think this works and is much simple. Anything I missed?


	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
	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 A2 and get

	xhist[ 0] = B64
	xhist[ 1] = A2   <-- idx
	xhist[ 2] = B2
	...
	xhist[63] = B63

and the commit_xhlocks() will happily create links between C and A2,
B2..B64.

The C<->A2 link is desired, the C<->B* are not.