linux-kernel - Re: [PATCH] sched: fix infinity loop in update_blocked

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Date:   Thu, 27 Dec 2018 17:53:52 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Xie XiuQi <xiexiuqi@...wei.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, xiezhipeng1@...wei.com,
        huawei.libin@...wei.com,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dmitry Adamushko <dmitry.adamushko@...il.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH] sched: fix infinity loop in update_blocked_averages

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