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

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Date:   Wed, 5 Jul 2017 20:50:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
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 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..

> +	 * 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.