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Date: Mon, 19 Jun 2017 22:55:37 +0200
From: "Jason A. Donenfeld" <>
To: Sebastian Andrzej Siewior <>
Cc: "Theodore Ts'o" <>, Linux Crypto Mailing List <>, 
	LKML <>,, 
	Greg Kroah-Hartman <>, Eric Biggers <>, 
	Linus Torvalds <>, David Miller <>, 
	Thomas Gleixner <>
Subject: Re: [PATCH] random: silence compiler warnings and fix race

On Mon, Jun 19, 2017 at 9:45 AM, Sebastian Andrzej Siewior
<> wrote:
> ehm. You sure? I simply delayed the lock-dropping _after_ the state
> variable was been modified. So it was basically what your patch did
> except it was unlocked later…

Yes, I'm sure. You moved the call to invalidate_batched_entropy() to
be after the assignment of crng_init. However, the call to
invalidate_batched_entropy() must be made _before_ the assignment of

>> > Are use about that? I am not sure that the gcc will inline "crng_init"
>> > read twice. It is not a local variable. READ_ONCE() is usually used
>> > where gcc could cache a memory access but you do not want this. But hey!
>> > If someone knows better I am here to learn.
>> The whole purpose is that I _want_ it to cache the memory access so
>> that it is _not_ inlined. So, based on your understanding, it does
>> exactly what I intended it to do. The reason is that I'd like to avoid
>> a lock imbalance, which could happen if the read is inlined.
> So it was good as it was which means you can drop that READ_ONCE().

Except READ_ONCE ensures that the compiler will never inline it, so it
actually needs to stay.

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