On Tue, Dec 04, 2007 at 02:45:06PM -0500, Mathieu Desnoyers wrote:
> * Andrew Morton ([email protected]) wrote:
> > On Tue, 4 Dec 2007 14:21:00 -0500
> > Mathieu Desnoyers <[email protected]> wrote:
> >
> > > > > + */
> > > > > +void marker_probe_cb(const struct marker *mdata, void *call_private,
> > > > > + const char *fmt, ...)
> > > > > +{
> > > > > + va_list args;
> > > > > + char ptype;
> > > > > +
> > > > > + preempt_disable();
> > > >
> > > > What are the preempt_disable()s doing in here?
> > > >
> > > > Unless I missed something obvious, a comment is needed here (at least).
> > > >
> > >
> > > They make sure the teardown of the callbacks can be done correctly when
> > > they are in modules and they insure RCU read coherency. Will add
> > > comment.
> >
> > So shouldn't it be using rcu_read_lock()? If that does not suit, should we
> > be adding new rcu primitives rather than open-coding and adding dependencies?
>
> Hrm, yes, good point. Since there seems to be extra magic under
> __acquire(RCU); and rcu_read_acquire();, the the fact that I use
> rcu_barrier() for synchronization, we should. I'll change it.
(Sorry to show up so late... It has been a bit crazy of late...)
The __acquire(RCU) and rcu_read_acquire() are strictly for the benefit
of sparse -- they allow it to detect mismatched rcu_read_lock() and
rcu_read_unlock() pairs. (Restricted to a single function, but so
it goes.)
I don't claim to fully understand this code, so may be way off base.
However, it looks like you are relying on stop_machine(), which in
turn interacts with preempt_disable(), but -not- necessarily with
rcu_read_lock(). Now, your rcu_barrier() call -does- interact with
rcu_read_lock() correctly, but either you need the preempt_disable()s
to interact correctly with stop_machine(), or you need to update the
comments calling out dependency on stop_machine().
Or it might be that the RCU API needs a bit of expanding. For example,
if you absolutely must use call_rcu(), and you also must absolutely
rely on stop_machine(), this might indicate that we need to add a
call_rcu_sched() as an asynchronous counterpart to synchronize_sched().
This would also require an rcu_sched_barrier() as well, to allow safe
unloading of modules using call_rcu_sched().
Or am I missing something?
Thanx, Paul
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