Re: [RFC v5 09/11] mm: Try spin lock in speculative path

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [RFC v5 09/11] mm: Try spin lock in speculative path
Date	2017-07-06 16:50 +0200
Message-ID	<u0ggq-67f-15@gated-at.bofh.it> (permalink)
References	<tT3Hj-3lN-3@gated-at.bofh.it> <tT3Hk-3lN-9@gated-at.bofh.it> <tZXGN-1Dt-9@gated-at.bofh.it> <u0fkn-4Ys-49@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Thu, Jul 06, 2017 at 03:46:59PM +0200, Laurent Dufour wrote:
> On 05/07/2017 20:50, Peter Zijlstra wrote:
> > On Fri, Jun 16, 2017 at 07:52:33PM +0200, Laurent Dufour wrote:
> >> @@ -2294,8 +2295,19 @@ static bool pte_map_lock(struct vm_fault *vmf)
> >>  	if (vma_has_changed(vmf->vma, vmf->sequence))
> >>  		goto out;
> >>  
> >> -	pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
> >> -				  vmf->address, &ptl);

> >> +	ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> >> +	pte = pte_offset_map(vmf->pmd, vmf->address);
> >> +	if (unlikely(!spin_trylock(ptl))) {
> >> +		pte_unmap(pte);
> >> +		goto out;
> >> +	}
> >> +
> >>  	if (vma_has_changed(vmf->vma, vmf->sequence)) {
> >>  		pte_unmap_unlock(pte, ptl);
> >>  		goto out;
> > 
> > Right, so if you look at my earlier patches you'll see I did something
> > quite disgusting here.
> > 
> > Not sure that wants repeating, but I cannot remember why I thought this
> > deadlock didn't exist anymore.
> 
> Regarding the deadlock I did face it on my Power victim node, so I guess it
> is still there, and the stack traces are quiet explicit.
> Am I missing something here ?

No, you are right in that the deadlock is quite real. What I cannot
remember is what made me think to remove the really 'wonderful' code I
had to deal with it.

That said, you might want to look at how often you terminate the
speculation because of your trylock failing. If that shows up at all we
might need to do something about it.

Thread

Re: [RFC v5 09/11] mm: Try spin lock in speculative path Peter Zijlstra <peterz@infradead.org> - 2017-07-05 21:00 +0200
  Re: [RFC v5 09/11] mm: Try spin lock in speculative path Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-07-06 15:50 +0200
    Re: [RFC v5 09/11] mm: Try spin lock in speculative path Peter Zijlstra <peterz@infradead.org> - 2017-07-06 16:50 +0200
      Re: [RFC v5 09/11] mm: Try spin lock in speculative path Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-07-06 17:30 +0200
        Re: [RFC v5 09/11] mm: Try spin lock in speculative path Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:20 +0200

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