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SubjectRe: [RFC v5 09/11] mm: Try spin lock in speculative path
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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);
>> + /* Same as pte_offset_map_lock() except that we call
>
> comment style..

Hi Peter and thanks for your work and review.

I'll fix this comment style.

>
>> + * spin_trylock() in place of spin_lock() to avoid race with
>> + * unmap path which may have the lock and wait for this CPU
>> + * to invalidate TLB but this CPU has irq disabled.
>> + * Since we are in a speculative patch, accept it could fail
>> + */
>> + 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 ?

Thanks,
Laurent.

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