linux-kernel - Re: [PATCH 3/4 v0.5] sched/umcg: RFC: implement UMCG syscalls

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Date:   Tue, 14 Sep 2021 09:51:28 -0700
From:   Peter Oskolkov <posk@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Peter Oskolkov <posk@...k.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Andrei Vagin <avagin@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [PATCH 3/4 v0.5] sched/umcg: RFC: implement UMCG syscalls

On Wed, Sep 8, 2021 at 6:40 PM Jann Horn <jannh@...gle.com> wrote:

[...]

> I think umcg_idle_loop() should never be called from scheduler
> callbacks (meaning umcg_wq_worker_running()), only from UMCG syscalls.

I'm moving umcg_wq_worker_running() out of
core.c/sched_update_worker() and into
/kernel/entry/common.c/exit_to_user_mode_loop()
(and will rename the function appropriately).
It seems rescheduling/sleeping there is fine.

I'm not yet sure if this is all that is needed to deal with
UMCG_TF_PREEMPTED flag; but I don't expect to see
any locks held when the task truly returns to the userspace.

Maybe I'll need to set TIF_NOTIFY_RESUME in sched_update_worker()...

[...]