Messages in this thread | | | Date | Tue, 29 Dec 2009 15:24:41 +0530 | From | Balbir Singh <> | Subject | Re: [RFC PATCH] asynchronous page fault. |
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* Peter Zijlstra <peterz@infradead.org> [2009-12-28 09:32:53]:
> On Mon, 2009-12-28 at 06:27 +0530, Balbir Singh wrote: > > * Peter Zijlstra <peterz@infradead.org> [2009-12-27 12:19:56]: > > > > > Your changelog states as much. > > > > > > "Even if RB-tree rotation occurs while we walk tree for look-up, we just > > > miss vma without oops." > > > > > > However, since this is the case, do we still need the > > > rcu_assign_pointer() conversion your patch does? All I can see it do is > > > slow down all RB-tree users, without any gain. > > > > Don't we need the rcu_assign_pointer() on the read side primarily to > > make sure the pointer is still valid and assignments (writes) are not > > re-ordered? Are you suggesting that the pointer assignment paths be > > completely atomic? > > rcu_assign_pointer() is the write side, but if you need a barrier, you > can make do with a single smp_wmb() after doing the rb-tree op. There is > no need to add multiple in the tree-ops themselves. >
Yes, that makes sense.
> You cannot make the assignment paths atomic (without locks) that's the > whole problem. >
True, but pre-emption can be nasty in some cases. But I am no expert in the atomicity of operations like assignments across architectures. I assume all word, long assignments are.
-- Balbir
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