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From:   Peter Oskolkov <posk@posk.io>
Date:   Sun, 11 Jul 2021 09:35:22 -0700
Message-ID: <CAFTs51WV6AD+ev4=RxXExjT3=TtaOZdE1avEY_uWHmUTb87mJw@mail.gmail.com>
Subject: Re: [RFC PATCH 3/3 v0.2] sched/umcg: RFC: implement UMCG syscalls
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-api@vger.kernel.org
Cc:     Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Peter Oskolkov <posk@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andrei Vagin <avagin@google.com>,
        Jim Newsome <jnewsome@torproject.org>,
        Jann Horn <jannh@google.com>
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On Thu, Jul 8, 2021 at 12:46 PM Peter Oskolkov <posk@posk.io> wrote:
[...]

> Pretty much everything works, with one issue: when a worker
> blocks, we need to wake its server in umcg_wq_worker_sleeping
> called from sched_submit_work within preempt_disable block.
> As the server_tid is set by the userspace, it can point to
> a running/spinning task, and so wake_server will hang waiting
> for ttwu() to succeed. I do not think this is appropriate,
> but I am not sure at the moment how to resolve this. Any sugestions?

[...]

I think I can solve this by carefully ordering state changes (both
umcg state and task state) and maybe sending a signal to the wakee if
not enough. I'll try this approach in v0.3.