Re: [PATCH] sched: reset sched_entity depth on changing parent

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 24, 2014 at 11:07:46AM -0400, Burke Libbey wrote:
> From 2014-02-15: https://lkml.org/lkml/2014/2/15/217
> 
> This issue was reported and patched, but it still occurs in some situations on
> newer kernel versions.
> 
> [2249353.328452] BUG: unable to handle kernel NULL pointer dereference at 0000000000000150
> [2249353.336528] IP: [<ffffffff810b1cf7>] check_preempt_wakeup+0xe7/0x210
> 
> se.parent gets out of sync with se.depth, causing a panic when the algorithm in
> find_matching_se assumes they are correct. This patch forces se.depth to be
> updated every time se.parent is, so they can no longer become desync'd.
> 
> CC: Ingo Molnar <mingo@kernel.org>
> CC: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Burke Libbey <burke.libbey@shopify.com>
> ---
> 
> I haven't been able to isolate the problem. Though I'm pretty confident this
> fixes the issue I've been having, I have not been able to prove it.

So this isn't correct, switching rq should not change depth. I suspect
you're just papering over the issue by frequently resetting the value,
which simply narrows the race window.