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? -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
