Re: linux-next: manual merge of the akpm-current tree with the tip tree
From: Peter Zijlstra
Date: Sun Aug 13 2017 - 08:50:39 EST
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 âanythingâ 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?