linux-kernel - Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

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Date:   Mon, 23 Sep 2019 17:23:03 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     John Hubbard <jhubbard@...dia.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Arnd Bergmann <arnd@...db.de>, Jason Gunthorpe <jgg@...pe.ca>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        YueHaibing <yuehaibing@...wei.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Keith Busch <keith.busch@...el.com>,
        Richard Fontana <rfontana@...hat.com>,
        Paul Mackerras <paulus@...ba.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Ganesh Goudar <ganeshgr@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Allison Randal <allison@...utok.net>
Subject: Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting
 method to skip serializing

On Mon, 2019-09-23 at 12:58 -0700, John Hubbard wrote:
> 
> CPU 0                            CPU 1
> ------                         --------------
>                                READ(pte) (re-ordered at run time)
>                                atomic_inc(val) (no run-time memory barrier!)
>                            
> pmd_clear(pte)
> if (val)
>     run_on_all_cpus(): IPI
>                                local_irq_disable() (also not a mem barrier)
> 
>                                if(pte)
>                                   walk page tables

Let me see if I can understand,
On most patches, it would be:

CPU 0                            CPU 1
------				--------------
				ptep = __find_linux_pte  
				(re-ordered at run time)
				atomic_inc(val) 
pmd_clear(pte)
smp_mb()
if (val)
    run_on_all_cpus(): IPI
                               local_irq_disable() 

                               if(ptep)
                                  pte = *ptep;

Is that what you meant?



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