linux-kernel - Re: [PATCH v2 3/6] hw_random: use reference counts on each struct hwrng.

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Date:	Mon, 20 Oct 2014 05:58:30 +0200
From:	Amos Kong <akong@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	virtualization@...ts.linux-foundation.org,
	herbert@...dor.apana.org.au, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, m@...s.ch, mpm@...enic.com,
	amit.shah@...hat.com
Subject: Re: [PATCH v2 3/6] hw_random: use reference counts on each struct
 hwrng.

On Mon, Oct 20, 2014 at 10:42:11AM +1030, Rusty Russell wrote:
> Amos Kong <akong@...hat.com> writes:
> > We got a warning in boot stage when above set_current_rng() is executed,
> > it can be fixed by init rng->ref in hwrng_init().
> >
> >
> > @@ -166,6 +169,8 @@ static inline int hwrng_init(struct hwrng *rng)
> >         if (current_quality > 0 && !hwrng_fill)
> >                 start_khwrngd();
> >  
> > +       kref_init(&rng->ref);
> > +
> >         return 0;
> >  }
> 
> OK, I folded this fix on.

Thanks.

Reviewed-by: Amos Kong <akong@...hat.com>
 
> Thanks,
> Rusty.
> 
> hw_random: use reference counts on each struct hwrng.
> 
> current_rng holds one reference, and we bump it every time we want
> to do a read from it.
> 
> This means we only hold the rng_mutex to grab or drop a reference,
> so accessing /sys/devices/virtual/misc/hw_random/rng_current doesn't
> block on read of /dev/hwrng.
> 
> Using a kref is overkill (we're always under the rng_mutex), but
> a standard pattern.
> 
> This also solves the problem that the hwrng_fillfn thread was
> accessing current_rng without a lock, which could change (eg. to NULL)
> underneath it.
> 
> v3: initialize kref (thanks Amos Kong)
> v2: fix missing put_rng() on exit path (thanks Amos Kong)
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
> 
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index a0905c818e13..0ecac38da954 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
....
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