Re: linux-next: manual merge of the akpm-current tree with the tip tree

[Nadav Amit <namit@vmware.com>]

Minchan Kim <minchan@kernel.org> wrote:

> On Sun, Aug 13, 2017 at 02:50:19PM +0200, Peter Zijlstra wrote:
>> 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?
> 
> No, because CPU 1 mofified pte and added it into tlb range
> so regardless of nested, it will flush TLB so there is no stale
> TLB problem.
> 
>> 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?
> 
> Ditto. Since CPU 1 has added range, it will flush TLB regardless
> of nested condition.
> 
>> 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?
> 
> mm_tlb_flush_nested aims for the CPU side where there is no pte update
> but need TLB flush.
> As I wrote https://urldefense.proofpoint.com/v2/url?u=https-3A__marc.info_-3Fl-3Dlinux-2Dmm-26m-3D150267398226529-26w-3D2&d=DwIDaQ&c=uilaK90D4TOVoH58JNXRgQ&r=x9zhXCtCLvTDtvE65-BGSA&m=v2Z7eDi7z1H9zdngcjZvlNeBudWzA9KvcXFNpU2A77s&s=amaSu_gurmBHHPcl3Pxfdl0Tk_uTnmf60tMQAsNDHVU&e= ,
> it has stable TLB problem if we don't flush TLB although there is no
> pte modification.

To clarify: the main problem that these patches address is when the first
CPU updates the PTE, and second CPU sees the updated value and thinks: “the
PTE is already what I wanted - no flush is needed”.

For some reason (I would assume intentional), all the examples here first
“do not modify” the PTE, and then modify it - which is not an “interesting”
case. However, based on what I understand on the memory barriers, I think
there is indeed a missing barrier before reading it in
mm_tlb_flush_nested(). IIUC using smp_mb__after_unlock_lock() in this case,
before reading, would solve the problem with least impact on systems with
strong memory ordering.

Minchan, as for the solution you proposed, it seems to open again a race,
since the “pending” indication is removed before the actual TLB flush is
performed.

Nadav