lkml.org 
[lkml]   [2010]   [May]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH RFC tip/core/rcu 04/23] net: Make accesses to ->br_port safe for sparse RCU
    On Wed, May 12, 2010 at 02:44:53PM -0700, Stephen Hemminger wrote:
    > On Wed, 12 May 2010 14:33:23 -0700
    > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
    >
    > > diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
    > > index 9101a4e..3f66cd1 100644
    > > --- a/net/bridge/br_fdb.c
    > > +++ b/net/bridge/br_fdb.c
    > > @@ -246,7 +246,7 @@ int br_fdb_test_addr(struct net_device *dev, unsigned char *addr)
    > > return 0;
    > >
    > > rcu_read_lock();
    > > - fdb = __br_fdb_get(dev->br_port->br, addr);
    > > + fdb = __br_fdb_get(br_port(dev)->br, addr);
    > > ret = fdb && fdb->dst->dev != dev &&
    > > fdb->dst->state == BR_STATE_FORWARDING;
    > > rcu_read_unlock();
    > > diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
    > > index 846d7d1..4fedb60 100644
    > > --- a/net/bridge/br_private.h
    > > +++ b/net/bridge/br_private.h
    > > @@ -229,6 +229,14 @@ static inline int br_is_root_bridge(const struct net_bridge *br)
    > > return !memcmp(&br->bridge_id, &br->designated_root, 8);
    > > }
    > >
    > > +static inline struct net_bridge_port *br_port(const struct net_device *dev)
    > > +{
    > > + if (!dev)
    > > + return NULL;
    > > +
    > > + return rcu_dereference(dev->br_port);
    > > +}
    >
    > Looks like this is wrapping existing problems, and hurting not helping.
    >
    > Why introduce a wrapper that could return NULL and not check the
    > result?

    Fair point!

    > I would rather that:
    > 1. dev should never be null in this cases so the first if() is
    > unnecessary, and confuses the semantics.
    > 2. don't use wrapper br_port()
    > 3. have callers check that rcu_dereference(dev->br_port) did not
    > return NULL.
    > If they derefernce does return NULL, then it means other CPU
    > has started tear down and this CPU should just go home quietly.

    OK.

    The reason for br_port() is to allow ->br_port to be a void*. If we
    eliminate br_port(), then it is necessary to make the definition of the
    struct net_bridge_port available everywhere that ->br_port is given to
    rcu_dereference(). The reason for this is that Arnd's sparse-based RCU
    checking code uses __rcu to tag the data pointed to by an RCU-protected
    pointer. This in turn means that rcu_dereference() and friends must
    now have access to the pointed-to type, as is done in patch 6 in this
    series.

    One way to make struct net_bridge_port available is to put:

    #include "../../net/bridge/br_private.h"

    in include/linux/netdevice.h.

    However, when I try this, I get lots of build errors, which was what led
    us to the path of making ->br_port be a void*, thus requiring the br_port()
    helper function in cases where the caller needs the underlying type.

    What should we be doing instead?

    Thanx, Paul


    \
     
     \ /
      Last update: 2010-05-13 00:37    [W:0.026 / U:1.736 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site