Messages in this thread |  | | Date | Sun, 13 Aug 2017 14:50:19 +0200 | From | Peter Zijlstra <> | Subject | Re: linux-next: manual merge of the akpm-current tree with the tip tree |
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On Sun, Aug 13, 2017 at 06:06:32AM +0000, Nadav Amit wrote: > > however mm_tlb_flush_nested() is a mystery, it appears to care about > > anything inside the range. For now rely on it doing at least _a_ PTL > > lock instead of taking _the_ PTL lock. > > It does not care about “anything” inside the range, but only on situations > in which there is at least one (same) PT that was modified by one core and > then read by the other. So, yes, it will always be _the_ same PTL, and not > _a_ PTL - in the cases that flush is really needed. > > The issue that might require additional barriers is that > inc_tlb_flush_pending() and mm_tlb_flush_nested() are called when the PTL is > not held. IIUC, since the release-acquire might not behave as a full memory > barrier, this requires an explicit memory barrier.
So I'm not entirely clear about this yet.
How about:
CPU0 CPU1
tlb_gather_mmu()
lock PTLn no mod unlock PTLn
tlb_gather_mmu()
lock PTLm mod include in tlb range unlock PTLm
lock PTLn mod unlock PTLn
tlb_finish_mmu() force = mm_tlb_flush_nested(tlb->mm); arch_tlb_finish_mmu(force);
... more ...
tlb_finish_mmu()
In this case you also want CPU1's mm_tlb_flush_nested() call to return true, right?
But even with an smp_mb__after_atomic() at CPU0's tlg_bather_mmu() you're not guaranteed CPU1 sees the increment. The only way to do that is to make the PTL locks RCsc and that is a much more expensive proposition.
What about:
CPU0 CPU1
tlb_gather_mmu()
lock PTLn no mod unlock PTLn
lock PTLm mod include in tlb range unlock PTLm
tlb_gather_mmu()
lock PTLn mod unlock PTLn
tlb_finish_mmu() force = mm_tlb_flush_nested(tlb->mm); arch_tlb_finish_mmu(force);
... more ...
tlb_finish_mmu()
Do we want CPU1 to see it here? If so, where does it end?
CPU0 CPU1
tlb_gather_mmu()
lock PTLn no mod unlock PTLn
lock PTLm mod include in tlb range unlock PTLm
tlb_finish_mmu() force = mm_tlb_flush_nested(tlb->mm);
tlb_gather_mmu()
lock PTLn mod unlock PTLn
arch_tlb_finish_mmu(force);
... more ...
tlb_finish_mmu()
This?
Could you clarify under what exact condition mm_tlb_flush_nested() must return true?
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