Messages in this thread | | | Date | Sun, 31 Aug 2008 19:45:02 +0200 | From | Manfred Spraul <> | Subject | Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation |
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Paul E. McKenney wrote: > Assuming that the ordering of processing pending irqs and marking the > CPU offline in cpu_online_mask can be resolved as noted above, it should > work fine -- if a CPU's bit is clear, we can safely ignore it. The race > can be resolved by checking the CPU's bit in force_quiescent_state(). > > Or am I missing something? > Yes, that would work: Rule 1: after CPU_DEAD, a cpu is gone. The cpu is quiet, rcu callbacks must be moved to other cpus, ... Rule 2: if a cpu is not listed in cpu_online_mask, then it can be considered as outside a read-side critical section.
The problem with rule 2 is that it means someone [force_quiescent_state()] must poll the cpu_online_mask and look for changes. I'd really prefer a notifier. CPU_DYING is nearly the correct thing, it only has to be moved down 3 lines ;-) (I want to kill the bitmaps, not add a hierarchical bitmap polling system!) > It is entirely possible that rcu_try_flip_waitack() and > rcu_try_flip_waitmb() need to check the AND of rcu_cpu_online_map and > cpu_online_map. If this really is a problem (and it might well be), > then the easiest fix is to check for cpu_is_offline(cpu) in both > rcu_try_flip_waitmb_needed() and rcu_try_flip_waitack_needed(), and > that in both versions of both functions. Thoughts? > I made a mistake, get_online_cpus() stores current, not a cpu number. Thus the described race it not possible. Perhaps there are other users that could deadlock. I don't know enough about the preempt algorithm, thus I can't confirm if your proposal would work or not.
-- Manfred
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