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From:   Vineeth Pillai <vpillai@digitalocean.com>
Date:   Fri, 31 May 2019 11:23:48 -0400
Message-ID: <CANaguZCxqsxmsF3pSvgT78bD9KC_qzqU1m2D6tg3KPUNJijdsw@mail.gmail.com>
Subject: Re: [RFC PATCH v3 10/16] sched: Core-wide rq->lock
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>
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>
> I'm confused, how doesn't this break the invariant above?
>
> That is, all CPUs must at all times agree on the value of rq_lockp(),
> and I'm not seeing how that is true with the above changes.
>
While fixing the crash in cpu online/offline, I was focusing on
maintaining the invariance
of all online cpus to agree on the value of rq_lockp(). Would it be
safe to assume that
rq and rq_lock would be used only after a cpu is onlined(sched:active)?.

To maintain the strict invariance, the sibling should also disable
core scheduling, but
we need to empty the rbtree before disabling it. I am trying to see
how to empty the
rbtree safely in the offline context.

Thanks,
Vineeth