linux-kernel - Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap

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Message-ID: <874kno9pr9.fsf@nanos.tec.linutronix.de>
Date:   Wed, 23 Sep 2020 22:55:54 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>, peterz@...radead.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Paul McKenney <paulmck@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
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        Will Deacon <will@...nel.org>,
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        <linux-snps-arc@...ts.infradead.org>,
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        "David S. Miller" <davem@...emloft.net>,
        linux-sparc <sparclinux@...r.kernel.org>
Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends

On Wed, Sep 23 2020 at 11:52, Steven Rostedt wrote:
> On Wed, 23 Sep 2020 10:40:32 +0200
> peterz@...radead.org wrote:
>
>> However, with migrate_disable() we can have each task preempted in a
>> migrate_disable() region, worse we can stack them all on the _same_ CPU
>> (super ridiculous odds, sure). And then we end up only able to run one
>> task, with the rest of the CPUs picking their nose.
>
> What if we just made migrate_disable() a local_lock() available for !RT?
>
> I mean make it a priority inheritance PER CPU lock.
>
> That is, no two tasks could do a migrate_disable() on the same CPU? If
> one task does a migrate_disable() and then gets preempted and the
> preempting task does a migrate_disable() on the same CPU, it will block
> and wait for the first task to do a migrate_enable().
>
> No two tasks on the same CPU could enter the migrate_disable() section
> simultaneously, just like no two tasks could enter a preempt_disable()
> section.
>
> In essence, we just allow local_lock() to be used for both RT and !RT.
>
> Perhaps make migrate_disable() an anonymous local_lock()?
>
> This should lower the SHC in theory, if you can't have stacked migrate
> disables on the same CPU.

I'm pretty sure this ends up in locking hell pretty fast and aside of
that it's not working for scenarios like:

     kmap_local();
       migrate_disable();
       ...

     copy_from_user()
        -> #PF
           -> schedule()

which brought us into that discussion in the first place. You would stop
any other migrate disable user from running until the page fault is
resolved...

Thanks,

        tglx