Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 27 Dec 2018 17:53:52 -0800
From:   Tejun Heo <tj@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Vincent Guittot <vincent.guittot@linaro.org>,
        Sargun Dhillon <sargun@sargun.me>,
        Xie XiuQi <xiexiuqi@huawei.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, xiezhipeng1@huawei.com,
        huawei.libin@huawei.com,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Dmitry Adamushko <dmitry.adamushko@gmail.com>,
        Rik van Riel <riel@surriel.com>
Subject: Re: [PATCH] sched: fix infinity loop in update_blocked_averages
Message-ID: <20181228015352.GG2509588@devbig004.ftw2.facebook.com>
References: <1545879866-27809-1-git-send-email-xiexiuqi@huawei.com>
 <CAKfTPtDRGULW3mdFzeyT=OBPdtPsz-6BG-=UGWbK6RQ7Lg8YpA@mail.gmail.com>
 <20181227102107.GA21156@linaro.org>
 <CAKfTPtD7O_DU5no36jaQziZDCvy=BMJk+tZbnQk2yVt=pzRfPw@mail.gmail.com>
 <CAMp4zn-bN=DdiKox-xZ++i2n_pgTA7Ms+1hBnAY2AbYv2iqXOA@mail.gmail.com>
 <CAKfTPtAmkHZ5XVHpf6rE8nrSPGzW_uO7a0tvH5JqDSpTzK9D=Q@mail.gmail.com>
 <CAHk-=wja7WN1sDz5_E7hf8c47CmX=yH8xc3h0zmmF8Y_fSR71A@mail.gmail.com>
 <20181228011524.GF2509588@devbig004.ftw2.facebook.com>
 <CAHk-=wj33F7LKQS3wCYJtb_yCzYhbjPzFqhVS_ZPPNOWTTMHFQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=wj33F7LKQS3wCYJtb_yCzYhbjPzFqhVS_ZPPNOWTTMHFQ@mail.gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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