On Wed, Nov 12, 2014 at 02:11:23PM +1030, Rusty Russell wrote:
> Amos Kong <akong@xxxxxxxxxx> writes:
> > From: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> >
> > 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.
> >
> > v4: decrease last reference for triggering the cleanup
>
> This doesn't make any sense:
>
> > +static void drop_current_rng(void)
> > +{
> > + struct hwrng *rng = current_rng;
> > +
> > + BUG_ON(!mutex_is_locked(&rng_mutex));
> > + if (!current_rng)
> > + return;
> > +
> > + /* release current_rng reference */
> > + kref_put(¤t_rng->ref, cleanup_rng);
> > + current_rng = NULL;
> > +
> > + /* decrease last reference for triggering the cleanup */
> > + kref_put(&rng->ref, cleanup_rng);
> > +}
>
> Why would it drop the refcount twice? This doesn't make sense.
>
> Hmm, because you added kref_init, which initializes the reference count
> to 1, you created this bug.
I saw some kernel code uses kref_* helper functions, the reference
conter is initialized to 1. Some code didn't use the helper functions
to increase/decrease the reference counter. So I will drop kref_init()
and the second kref_put().
> Leave out the kref_init, and let it naturally be 0 (until, and if, it
> becomes current_rng). Add a comment if you want.
OK, thanks.
> Thanks,
> Rusty.
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--
Amos.
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