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

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Message-Id: <3af22f3b-03ab-1d37-b2b1-b616adde7eb6@linux.vnet.ibm.com>
Date:   Thu, 6 Jul 2017 15:46:59 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        kirill@...temov.name, ak@...ux.intel.com, mhocko@...nel.org,
        dave@...olabs.net, jack@...e.cz,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [RFC v5 09/11] mm: Try spin lock in speculative path

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.