On Thu, Sep 29, 2005 at 04:30:28PM -0700, Suzanne Wood wrote:
> > Date: Fri, 30 Sep 2005 07:28:36 +1000
> > From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
>
> > On Thu, Sep 29, 2005 at 09:02:29AM -0700, Suzanne Wood wrote:
> > >
> > > The exchange below suggests that it is equally important
> > > to have the rcu_dereference() in __in_dev_get(), so the
> > > idea of the only difference between in_dev_get and
> > > __in_dev_get being the refcnt may be accepted.
>
> > With __in_dev_get() it's the caller's responsibility to ensure
> > that RCU works correctly. Therefore if any rcu_dereference is
> > needed it should be done by the caller.
>
> This sounds reasonable to me. Does everyone agree?
Is there any case where __in_dev_get() might be called without
needing to be wrapped with rcu_dereference()? If so, then I
agree (FWIW, given my meagre knowledge of Linux networking).
If all __in_dev_get() invocations need to be wrapped in
rcu_dereference(), then it seems to me that there would be
motivation to bury rcu_dereference() in __in_dev_get().
> > Some callers of __in_dev_get() don't need rcu_dereference at all
> > because they're protected by the rtnl.
>
> > BTW, could you please move the rcu_dereference in in_dev_get()
> > into the if clause? The barrier is not needed when ip_ptr is
> > NULL.
>
> The trouble with that may be that there are three events, the
> dereference, the assignment, and the conditional test. The
> rcu_dereference() is meant to assure deferred destruction
> throughout.
One only needs an rcu_dereference() once on the data-flow path from
fetching the RCU-protected pointer to dereferencing that pointer.
If the pointer is NULL, there is no way you can dereference it,
so, technically, Herbert is quite correct.
However, rcu_dereference() only generates a memory barrier on DEC
Alpha, so there is normally no penalty for using it in the NULL-pointer
case. So, when using rcu_dereference() unconditionally simplifies
the code, it may make sense to "just do it".
Thanx, Paul
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