Re: [PATCH] drivers/crypto: Use RCU_INIT_POINTER(x, NULL) in nx/nx-842.c

[Arend van Spriel]

On 24/03/14 12:16, Neil Horman wrote:
> On Mon, Mar 24, 2014 at 01:01:04AM +0530, Monam Agarwal wrote:
> > This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)
> >
> > The rcu_assign_pointer() ensures that the initialization of a structure
> > is carried out before storing a pointer to that structure.
> > And in the case of the NULL pointer, there is no structure to initialize.
> > So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)
> >
> > Signed-off-by: Monam Agarwal <monamagarwal123@xxxxxxxxx>
> > ---
> >   drivers/crypto/nx/nx-842.c |    4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/crypto/nx/nx-842.c b/drivers/crypto/nx/nx-842.c
> > index 1e5481d..c4fcbf4 100644
> > --- a/drivers/crypto/nx/nx-842.c
> > +++ b/drivers/crypto/nx/nx-842.c
> > @@ -1234,7 +1234,7 @@ static int __exit nx842_remove(struct vio_dev *viodev)
> >   	old_devdata = rcu_dereference_check(devdata,
> >   			lockdep_is_held(&devdata_mutex));
> >   	of_reconfig_notifier_unregister(&nx842_of_nb);
> > -	rcu_assign_pointer(devdata, NULL);
> > +	RCU_INIT_POINTER(devdata, NULL);
> >   	spin_unlock_irqrestore(&devdata_mutex, flags);
> >   	synchronize_rcu();
> >   	dev_set_drvdata(&viodev->dev, NULL);
> > @@ -1285,7 +1285,7 @@ static void __exit nx842_exit(void)
> >   	spin_lock_irqsave(&devdata_mutex, flags);
> >   	old_devdata = rcu_dereference_check(devdata,
> >   			lockdep_is_held(&devdata_mutex));
> > -	rcu_assign_pointer(devdata, NULL);
> > +	RCU_INIT_POINTER(devdata, NULL);
> >   	spin_unlock_irqrestore(&devdata_mutex, flags);
> >   	synchronize_rcu();
> >   	if (old_devdata)
>
> This really does't seem right.  rcu_assign_pointer users ACCESS_ONCE to protect
> against multiple loads that allow for concurrent read/write use with parallel
> rcu_dereference calls, whereas RCU_INIT_POINTER does not.  It also just doesn't
> look right.  We've already initalized the pointerin nx842_init, we don't need to
> re-initalize it here, we need to assign it to NULL.

I was being cautious as well, but Paul gave me an explanation [1]

Hope it helps you as well ;-)

But I think I also saw a patch that modifies rcu_assign_pointer to 
handle NULL assignment. Don't recall the thread.

Regards,
Arend

[1] http://mid.gmane.org/20140320150601.GK4405@xxxxxxxxxxxxxxxxxx

> Neil
>
> > --
> > 1.7.9.5
> >
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