Re: [PATCH] random: silence compiler warnings and fix race

[Jason A. Donenfeld]

On Mon, Jun 19, 2017 at 9:45 AM, Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> 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 laterâ

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
crng_init.

>> > 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.