linux-kernel - Re: [PATCH] /dev/random: Insufficient of entropy on many architectures

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Date:	Tue, 10 Sep 2013 12:38:56 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	LKML <linux-kernel@...r.kernel.org>, dave.taht@...ferbloat.net
Subject: Re: [PATCH] /dev/random: Insufficient of entropy on many architectures

On Tue, Sep 10, 2013 at 4:31 AM, Stephan Mueller <smueller@...onox.de> wrote:
> Hi,
>
> /dev/random uses the get_cycles() function to obtain entropy in addition to jiffies and the event value of hardware events.
>
> Typically the high-resolution timer of get_cycles delivers the majority of entropy, because the event value is quite deterministic and jiffies are very coarse.
>
[snip]
> The following patch uses the clocksource clock for a time value in case get_cycles returns 0. As clocksource may not be available during boot time, a flag is introduced which allows random.c to check the availability of clocksource.
>
[snip]
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 48b9fff..75b1613 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -35,6 +35,7 @@ static struct timekeeper timekeeper;
>  static DEFINE_RAW_SPINLOCK(timekeeper_lock);
>  static seqcount_t timekeeper_seq;
>  static struct timekeeper shadow_timekeeper;
> +static bool timekeeper_enabled = 0;
>
>  /* flag for if timekeeping is suspended */
>  int __read_mostly timekeeping_suspended;
> @@ -833,8 +834,15 @@ void __init timekeeping_init(void)
>
>         write_seqcount_end(&timekeeper_seq);
>         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
> +       timekeeper_enabled = 1;
>  }
>

So the end of timekeeping_init() may not be what you want here. This
only means we've started up the timekeping core with only the default
clocksource (with only few exceptions, this is almost always jiffies).
Then as clocksource drivers are initialized, they are registered and
the timekeeping core will switch over to the best available
clocksource.  Also, to avoid the churn at boot of switching to every
clocksource registered, we queue them up and wait until fs_init time
to switch to whatever is the best available then.

So its likely with this patch that the systems all still end up using
jiffies for their clocksource at least until fs_init time.

thanks
-john
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