Messages in this thread | | | Date | Thu, 20 Dec 2007 09:25:40 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [patch 1/2] Linux Kernel Markers - Support Multiple Probes |
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* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > 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? >
Hi Paul,
Sorry about the late response; I was away for small vacation :)
Yes, I need both :
- disabling preemption at marker site is required to protect against deletion of probe code when modules are unloaded. - I use the call_rcu() to execute delayed free of my data structures. I could do all that synchronously with synchronize_sched(), but batch registration/unregistration would be just too slow. I don't want to take a few minutes to activate ~100 probes, that would be insane.
So yes, adding the new piece of API sounds like a good idea. Meanwhile, I guess I could just do this in the code executed around probe call, although it has a performance impact :
rcu_read_lock(); preempt_disable();
probe_call();
preempt_enable(); rcu_read_unlock();
Thanks very much for the review,
Mathieu
> Thanx, Paul
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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