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

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On 2017-06-17 02:39:40 [+0200], Jason A. Donenfeld wrote:
> On Fri, Jun 16, 2017 at 4:35 PM, Sebastian Andrzej Siewior
> <bigeasy@linutronix.de> wrote:
> > I wouldn't just push the lock one up as is but move that write part to
> > crng_init to remain within the locked section. Like that:
> 
> We can't quite do that, because invalidate_batched_entropy() needs to
> be called _before_ crng_init. Otherwise a concurrent call to
> get_random_u32/u64() will have crng_init being the wrong value when
> the batched entropy is still old.

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…

> 
> > 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().

> Jason

Sebastian

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[PATCH v5 00/13] Unseeded In-Kernel Randomness Fixes "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 13/13] random: warn when kernel uses unseeded randomness "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 06/13] iscsi: ensure RNG is seeded before use "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 05/13] crypto/rng: ensure that the RNG is ready before using "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 11/13] net/route: use get_random_int for random counter "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 10/13] net/neighbor: use get_random_u32 for 32-bit hash random "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 08/13] cifs: use get_random_u32 for 32-bit lock random "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 12/13] bluetooth/smp: ensure RNG is properly seeded before ECDH use "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 02/13] random: add synchronous API for the urandom pool "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 01/13] random: invalidate batched entropy after crng init "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
    Re: [PATCH v5 01/13] random: invalidate batched entropy after crng  init Sebastian Andrzej Siewior <sebastian@breakpoint.cc> - 2017-06-14 21:40 +0200
      Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-15 00:40 +0200
        Re: [PATCH v5 01/13] random: invalidate batched entropy after crng  init Sebastian Andrzej Siewior <sebastian@breakpoint.cc> - 2017-06-16 10:40 +0200
          Re: [PATCH v5 01/13] random: invalidate batched entropy after crng init "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-16 14:20 +0200
            Re: [PATCH v5 01/13] random: invalidate batched entropy after crng  init Sebastian Andrzej Siewior <sebastian@breakpoint.cc> - 2017-06-16 16:40 +0200
      [PATCH] random: silence compiler warnings and fix race "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-15 00:50 +0200
        Re: [PATCH] random: silence compiler warnings and fix race Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-06-16 16:40 +0200
          Re: [PATCH] random: silence compiler warnings and fix race "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-17 02:40 +0200
            Re: [PATCH] random: silence compiler warnings and fix race Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-06-19 09:50 +0200
              Re: [PATCH] random: silence compiler warnings and fix race "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-19 23:00 +0200
        Re: [PATCH] random: silence compiler warnings and fix race "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-19 23:00 +0200
  [PATCH v5 03/13] random: add get_random_{bytes,u32,u64,int,long,once}_wait family "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200
  [PATCH v5 04/13] security/keys: ensure RNG is seeded before use "Jason A. Donenfeld" <Jason@zx2c4.com> - 2017-06-08 01:30 +0200

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