linux-kernel - Re: [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER

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Date:   Tue, 15 Sep 2020 21:04:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER_ATOMIC unconditionally

On Tue, Sep 15 2020 at 20:10, Herbert Xu wrote:
> On Tue, Sep 15, 2020 at 01:08:31PM +0300, Ard Biesheuvel wrote:
>>
>> But making atomic kmap preemptible/sleepable creates the exact same
>> problem, i.e., that we have no idea which existing callers are
>> currently relying on those preemption disabling semantics, so we can't
>> just take them away. Or am I missing something?
>
> Good point.
>
> Thomas mentioned that RT has been doing this for a while now so
> perhaps someone has studied this problem already? Thomas?

RT is substituting preempt_disable() with migrate_disable() which pins
the task on the CPU so that per CPU stuff still works. And we did quite
some staring whether there is code which purely relies on the
preempt_disable() to prevent reentrancy, but there is almost none.

Though we don't have migrate disable on !RT and PeterZ is not a great
fan of making it available as it wreckages schedulability - though IMO
not much more than preempt disable :)

Thanks,

        tglx