Re: [PATCH 1/2] rcu: add rcu_access_pointer and rcu_dereference_protect

Paul E. McKenney
2010-04-07T15:59:35Z

On Wed, Apr 07, 2010 at 04:56:50PM +0200, Eric Dumazet wrote:
> Le mercredi 07 avril 2010 =E0 14:57 +0100, David Howells a =E9crit :
> > From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >=20
> > This patch adds variants of rcu_dereference() that handle situation=
s
> > where the RCU-protected data structure cannot change, perhaps due t=
o
> > our holding the update-side lock, or where the RCU-protected pointe=
r is
> > only to be fetched, not dereferenced.
> >=20
> > The new rcu_access_pointer() primitive is for the case where the po=
inter
> > is be fetch and not dereferenced. This primitive may be used witho=
ut
> > protection, RCU or otherwise, due to the fact that it uses ACCESS_O=
NCE().
> >=20
> > The new rcu_dereference_protect() primitive is for the case where u=
pdates
> > are prevented, for example, due to holding the update-side lock. T=
his
> > primitive does neither ACCESS_ONCE() nor smp_read_barrier_depends()=
, so
> > can only be used when updates are somehow prevented.
> >=20
> > Suggested-by: David Howells <dhowells@redhat.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Signed-off-by: David Howells <dhowells@redhat.com>
> > ---
> >=20
> > include/linux/rcupdate.h | 34 ++++++++++++++++++++++++++++++++++
> > 1 files changed, 34 insertions(+), 0 deletions(-)
> >=20
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 872a98e..a1b14b6 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -209,9 +209,43 @@ static inline int rcu_read_lock_sched_held(voi=
d)
> > rcu_dereference_raw(p); \
> > })
> > =20
> > +/**
> > + * rcu_access_pointer - fetch RCU pointer with no dereferencing
> > + *
> > + * Return the value of the specified RCU-protected pointer, but om=
it the
> > + * smp_read_barrier_depends() and keep the ACCESS_ONCE(). This is=
useful
> > + * when the value of this pointer is accessed, but the pointer is =
not
> > + * dereferenced, for example, when testing an RCU-protected pointe=
r against
> > + * NULL. This may also be used in cases where update-side locks p=
revent
> > + * the value of the pointer from changing, but rcu_dereference_pro=
tect()
> > + * is a lighter-weight primitive for this use case.
> > + */
> > +#define rcu_access_pointer(p) \
> > + ({ \
> > + ACCESS_ONCE(p); \
> > + })
> > +
> > +/**
> > + * rcu_dereference_protected - fetch RCU pointer when updates prev=
ented
> > + *
> > + * Return the value of the specified RCU-protected pointer, but om=
it
> > + * both the smp_read_barrier_depends() and the ACCESS_ONCE(). Thi=
s
> > + * is useful in cases where update-side locks prevent the value of=
the
> > + * pointer from changing. Please note that this primitive does -n=
ot-
> > + * prevent the compiler from repeating this reference or combining=
it
> > + * with other references, so it should not be used without protect=
ion
> > + * of appropriate locks.
> > + */
> > +#define rcu_dereference_protected(p) \
> > + ({ \
> > + (p); \
> > + })
> > +
> > #else /* #ifdef CONFIG_PROVE_RCU */
> > =20
> > #define rcu_dereference_check(p, c) rcu_dereference_raw(p)
> > +#define rcu_access_pointer(p) ACCESS_ONCE(p)
> > +#define rcu_dereference_protect(p) (p)
> > =20
> > #endif /* #else #ifdef CONFIG_PROVE_RCU */
> > =20
> >=20
> > --
>=20
> This is not the version Paul posted.=20

I blew the name -- rcu_dereference_protected() is in fact a better name=
=2E

> Removing checks just to shutup warnings ?
>=20
> All the point is to get lockdep assistance, and you throw it away.
>=20
> We want to explicit the condition, so that RCU users can explicitly
> state what protects their data.

What Eric said!!! ;-)

Thanx, Paul