Messages in this thread | | | Date | Mon, 17 Dec 2007 09:40:23 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [patch 1/2] Linux Kernel Markers - Support Multiple Probes |
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On Tue, Dec 04, 2007 at 02:45:06PM -0500, Mathieu Desnoyers wrote: > * Andrew Morton (akpm@linux-foundation.org) wrote: > > On Tue, 4 Dec 2007 14:21:00 -0500 > > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> 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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