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From: Leonardo Bras <leonardo@linux.ibm.com>
To: John Hubbard <jhubbard@nvidia.com>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	Linux-MM <linux-mm@kvack.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Richard Fontana <rfontana@redhat.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	YueHaibing <yuehaibing@huawei.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Keith Busch <keith.busch@intel.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, Paul Mackerras <paulus@samba.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Allison Randal <allison@lohutok.net>,
	Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>,
	Ganesh Goudar <ganeshgr@linux.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ira Weiny <ira.weiny@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing
Date: Mon, 23 Sep 2019 16:40:39 -0300	[thread overview]
Message-ID: <dc9fad3577551d34ead36c0f7340a573086c0cab.camel@linux.ibm.com> (raw)
In-Reply-To: <8706a1f1-0c5e-d152-938b-f355b9a5aaa8@nvidia.com>

[-- Attachment #1: Type: text/plain, Size: 1990 bytes --]

On Mon, 2019-09-23 at 11:14 -0700, John Hubbard wrote:
> On 9/23/19 10:25 AM, Leonardo Bras wrote:
> [...]
> That part is all fine, but there are no run-time memory barriers in the 
> atomic_inc() and atomic_dec() additions, which means that this is not
> safe, because memory operations on CPU 1 can be reordered. It's safe
> as shown *if* there are memory barriers to keep the order as shown:
> 
> CPU 0                            CPU 1
> ------                         --------------
>                                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)
> 
>                                READ(pte)
>                                if(pte)
>                                   walk page tables
> 
>                                local_irq_enable() (still not a barrier)
>                                atomic_dec(val)
> 
> free(pte)
> 
> thanks,

This is serialize:

void serialize_against_pte_lookup(struct mm_struct *mm)
{
	smp_mb();
	if (running_lockless_pgtbl_walk(mm))
		smp_call_function_many(mm_cpumask(mm), do_nothing,
NULL, 1);
}

That would mean:

CPU 0                            CPU 1
------                         --------------
                               atomic_inc(val) 
pmd_clear(pte)
smp_mb()
if (val)
    run_on_all_cpus(): IPI
                               local_irq_disable() 

                               READ(pte)
                               if(pte)
                                  walk page tables

                               local_irq_enable() (still not a barrier)
                               atomic_dec(val)

By https://www.kernel.org/doc/Documentation/memory-barriers.txt :
'If you need all the CPUs to see a given store at the same time, use
smp_mb().'

Is it not enough? 
Do you suggest adding 'smp_mb()' after atomic_{inc,dec} ?

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WARNING: multiple messages have this Message-ID (diff)
From: Leonardo Bras <leonardo@linux.ibm.com>
To: John Hubbard <jhubbard@nvidia.com>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	Linux-MM <linux-mm@kvack.org>
Cc: Dan Williams <dan.j.williams@intel.com>,
	Arnd Bergmann <arnd@arndb.de>, Jason Gunthorpe <jgg@ziepe.ca>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>,
	YueHaibing <yuehaibing@huawei.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Keith Busch <keith.busch@intel.com>,
	Richard Fontana <rfontana@redhat.com>,
	Paul Mackerras <paulus@samba.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Ganesh Goudar <ganeshgr@linux.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ira Weiny <ira.weiny@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Allison Randal <allison@lohutok.net>
Subject: Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing
Date: Mon, 23 Sep 2019 16:40:39 -0300	[thread overview]
Message-ID: <dc9fad3577551d34ead36c0f7340a573086c0cab.camel@linux.ibm.com> (raw)
In-Reply-To: <8706a1f1-0c5e-d152-938b-f355b9a5aaa8@nvidia.com>

[-- Attachment #1: Type: text/plain, Size: 1990 bytes --]

On Mon, 2019-09-23 at 11:14 -0700, John Hubbard wrote:
> On 9/23/19 10:25 AM, Leonardo Bras wrote:
> [...]
> That part is all fine, but there are no run-time memory barriers in the 
> atomic_inc() and atomic_dec() additions, which means that this is not
> safe, because memory operations on CPU 1 can be reordered. It's safe
> as shown *if* there are memory barriers to keep the order as shown:
> 
> CPU 0                            CPU 1
> ------                         --------------
>                                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)
> 
>                                READ(pte)
>                                if(pte)
>                                   walk page tables
> 
>                                local_irq_enable() (still not a barrier)
>                                atomic_dec(val)
> 
> free(pte)
> 
> thanks,

This is serialize:

void serialize_against_pte_lookup(struct mm_struct *mm)
{
	smp_mb();
	if (running_lockless_pgtbl_walk(mm))
		smp_call_function_many(mm_cpumask(mm), do_nothing,
NULL, 1);
}

That would mean:

CPU 0                            CPU 1
------                         --------------
                               atomic_inc(val) 
pmd_clear(pte)
smp_mb()
if (val)
    run_on_all_cpus(): IPI
                               local_irq_disable() 

                               READ(pte)
                               if(pte)
                                  walk page tables

                               local_irq_enable() (still not a barrier)
                               atomic_dec(val)

By https://www.kernel.org/doc/Documentation/memory-barriers.txt :
'If you need all the CPUs to see a given store at the same time, use
smp_mb().'

Is it not enough? 
Do you suggest adding 'smp_mb()' after atomic_{inc,dec} ?

[-- Attachment #2: This is a digitally signed message part --]
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  reply	other threads:[~2019-09-23 19:41 UTC|newest]

Thread overview: 72+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-20 19:50 [PATCH v2 00/11] Introduces new count-based method for monitoring lockless pagetable wakls Leonardo Bras
2019-09-20 19:50 ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 01/11] powerpc/mm: Adds counting method to monitor lockless pgtable walks Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-23 20:42   ` John Hubbard
2019-09-23 20:42     ` John Hubbard
2019-09-23 20:50     ` Leonardo Bras
2019-09-23 20:50       ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 02/11] asm-generic/pgtable: Adds dummy functions " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-23 20:39   ` John Hubbard
2019-09-23 20:39     ` John Hubbard
2019-09-23 20:48     ` Leonardo Bras
2019-09-23 20:48       ` Leonardo Bras
2019-09-23 20:53       ` John Hubbard
2019-09-23 20:53         ` John Hubbard
2019-09-20 19:50 ` [PATCH v2 03/11] mm/gup: Applies counting method to monitor gup_pgd_range Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-23 20:27   ` John Hubbard
2019-09-23 20:27     ` John Hubbard
2019-09-23 21:01     ` Leonardo Bras
2019-09-23 21:01       ` Leonardo Bras
2019-09-23 21:09       ` John Hubbard
2019-09-23 21:09         ` John Hubbard
2019-09-20 19:50 ` [PATCH v2 04/11] powerpc/mce_power: Applies counting method to monitor lockless pgtbl walks Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 05/11] powerpc/perf: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 06/11] powerpc/mm/book3s64/hash: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 07/11] powerpc/kvm/e500: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 08/11] powerpc/kvm/book3s_hv: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-23 20:47   ` John Hubbard
2019-09-23 20:47     ` John Hubbard
2019-09-20 19:50 ` [PATCH v2 09/11] powerpc/kvm/book3s_64: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 10/11] powerpc/book3s_64: Enables counting method to monitor lockless pgtbl walk Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 20:11   ` John Hubbard
2019-09-20 20:28     ` Leonardo Bras
2019-09-20 20:28       ` Leonardo Bras
2019-09-20 21:15       ` John Hubbard
2019-09-21  0:48       ` John Hubbard
2019-09-21  0:48         ` John Hubbard
2019-09-23 17:25         ` Leonardo Bras
2019-09-23 17:25           ` Leonardo Bras
2019-09-23 18:14           ` John Hubbard
2019-09-23 18:14             ` John Hubbard
2019-09-23 19:40             ` Leonardo Bras [this message]
2019-09-23 19:40               ` Leonardo Bras
2019-09-23 19:58               ` John Hubbard
2019-09-23 19:58                 ` John Hubbard
2019-09-23 20:23                 ` Leonardo Bras
2019-09-23 20:23                   ` Leonardo Bras
2019-09-23 20:26                   ` John Hubbard
2019-09-23 20:26                     ` John Hubbard
2019-09-20 19:56 ` [PATCH v2 00/11] Introduces new count-based method for monitoring lockless pagetable wakls Leonardo Bras
2019-09-20 19:56   ` Leonardo Bras
2019-09-20 20:12 ` Leonardo Bras
2019-09-20 20:12   ` Leonardo Bras
2019-09-20 21:24   ` John Hubbard
2019-09-20 21:24     ` John Hubbard
2019-09-23 20:51   ` John Hubbard
2019-09-23 20:51     ` John Hubbard
2019-09-23 20:58     ` Leonardo Bras
2019-09-23 20:58       ` Leonardo Bras
2019-09-24 21:23 [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing Leonardo Bras
2019-09-24 21:23 ` Leonardo Bras

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