linux-kernel - Re: [PATCH v3] random: remove batched entropy locking

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Date:   Wed, 16 Feb 2022 21:01:19 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        "Theodore Ts'o" <tytso@....edu>, Andy Lutomirski <luto@...nel.org>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3] random: remove batched entropy locking

On Fri, Feb 4, 2022 at 4:57 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
> On 2022-02-04 16:51:42 [+0100], Jason A. Donenfeld wrote:
> > index 455615ac169a..3e54b90a3ff8 100644
> > --- a/drivers/char/random.c
> > +++ b/drivers/char/random.c
> > @@ -1759,41 +1762,54 @@ u64 get_random_u64(void)
> >       unsigned long flags;
> >       struct batched_entropy *batch;
> >       static void *previous;
> > +     int next_gen;
> >
> >       warn_unseeded_randomness(&previous);
> >
> > -     batch = raw_cpu_ptr(&batched_entropy_u64);
> > -     spin_lock_irqsave(&batch->batch_lock, flags);
> > -     if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
> > +     batch = this_cpu_ptr(&batched_entropy_u64);
> > +     local_lock_irqsave(&batch->lock, flags);
>
> Does this compile and work? From the looks of it, this should be:
>
>         local_lock_irqsave(&batched_entropy_u64.lock, flags);
>         batch = this_cpu_ptr(&batched_entropy_u64);
>
> and we could do s/this_cpu_ptr/raw_cpu_ptr/

Why raw_cpu_ptr? include/linux/percpu-defs.h says about raw_*() operations:

 * Operations for contexts where we do not want to do any checks for
 * preemptions.  Unless strictly necessary, always use [__]this_cpu_*()
 * instead.

So when I see a raw_*() percpu thing, I read it as "it is expected
that we can migrate after this point (or we're in some really weird
context where the normal context check doesn't work)". Is that
incorrect?