Re: [PATCH] sched: fix infinity loop in update_blocked_averages

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Dec 27, 2018 at 05:36:47PM -0800, Linus Torvalds wrote:
> > Unless I'm totally confused, which is definitely possible, I don't
> > think there's a race condition and the only bug is the
> > tmp_alone_branch pointer getting dangled, which maybe doesn't happen
> > all that much?
> 
> Ahh. That would explain the list corruption. The next
> list_add_leaf_cfs_rq() could try to add to a removed entry.
> 
> How would you reset it? Do something like
> 
>        rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;
> 
> for every removal, or make it conditional on it matching the removed entry?

Vincent knows that part way better than me but I think the safest way
would be doing the optimization removal iff tmp_alone_branch is
already pointing to leaf_cfs_rq_list.  IIUC, it's pointing to
something else only while a branch is being built and deferring
optimization removal by an avg update cycle isn't gonna make any
difference anyway.

Thanks.

-- 
tejun