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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Fri, 21 Feb 2020 21:08:19 +0100
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Subject: Re: [PATCH] random: always use batched entropy for get_random_u{32,64}
To:     Tony Luck <tony.luck@gmail.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Ted Ts'o" <tytso@mit.edu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Feb 20, 2020 at 11:29 PM Tony Luck <tony.luck@gmail.com> wrote:
>
> Also ... what's the deal with a spin_lock on a per-cpu structure?
>
>         batch = raw_cpu_ptr(&batched_entropy_u64);
>         spin_lock_irqsave(&batch->batch_lock, flags);
>         if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
>                 extract_crng((u8 *)batch->entropy_u64);
>                 batch->position = 0;
>         }
>         ret = batch->entropy_u64[batch->position++];
>         spin_unlock_irqrestore(&batch->batch_lock, flags);
>
> Could we just disable interrupts and pre-emption around the entropy extraction?

Probably, yes... We can address this in a separate patch.