From: John Hubbard <jhubbard@nvidia.com>
To: Leonardo Bras <leonardo@linux.ibm.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 11:14:39 -0700 [thread overview]
Message-ID: <8706a1f1-0c5e-d152-938b-f355b9a5aaa8@nvidia.com> (raw)
In-Reply-To: <18c5c378db98f223a0663034baa9fd6ce42f1ec7.camel@linux.ibm.com>
On 9/23/19 10:25 AM, Leonardo Bras wrote:
> On Fri, 2019-09-20 at 17:48 -0700, John Hubbard wrote:
>>
> [...]
>> So it seems that full memory barriers (not just compiler barriers) are required.
>> If the irq enable/disable somehow provides that, then your new code just goes
>> along for the ride and Just Works. (You don't have any memory barriers in
>> start_lockless_pgtbl_walk() / end_lockless_pgtbl_walk(), just the compiler
>> barriers provided by the atomic inc/dec.)
>>
>> So it's really a pre-existing question about the correctness of the gup_fast()
>> irq disabling approach.
>
> I am not experienced in other archs, and I am still pretty new to
> Power, but by what I could understand, this behavior is better
> explained in serialize_against_pte_lookup.
>
> What happens here is that, before doing a THP split/collapse, the
> function does a update of the pmd and a serialize_against_pte_lookup,
> in order do avoid a invalid output on a lockless pagetable walk.
>
> Serialize basically runs a do_nothing in every cpu related to the
> process, and wait for it to return.
>
> This running depends on interrupt being enabled, so disabling it before
> gup_pgd_range() and re-enabling after the end, makes the THP
> split/collapse wait for gup_pgd_range() completion in every cpu before
> continuing. (here happens the lock)
>
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,
--
John Hubbard
NVIDIA
> (As told before, every gup_pgd_range() that occurs after it uses a
> updated pmd, so no problem.)
>
> I am sure other archs may have a similar mechanism using
> local_irq_{disable,enable}.
>
> Did it answer your questions?
>
> Best regards,
>
> Leonardo Bras
>
next prev parent reply other threads:[~2019-09-23 18:17 UTC|newest]
Thread overview: 37+ 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 ` [PATCH v2 01/11] powerpc/mm: Adds counting method to monitor lockless pgtable walks Leonardo Bras
2019-09-23 20:42 ` John Hubbard
2019-09-23 20:50 ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 02/11] asm-generic/pgtable: Adds dummy functions " Leonardo Bras
2019-09-23 20:39 ` John Hubbard
2019-09-23 20:48 ` Leonardo Bras
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
2019-09-23 20:27 ` John Hubbard
2019-09-23 21:01 ` Leonardo Bras
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 ` [PATCH v2 05/11] powerpc/perf: " Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 06/11] powerpc/mm/book3s64/hash: " Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 07/11] powerpc/kvm/e500: " Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 08/11] powerpc/kvm/book3s_hv: " Leonardo Bras
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 ` [PATCH v2 10/11] powerpc/book3s_64: Enables counting method to monitor lockless pgtbl walk 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 20:11 ` John Hubbard
2019-09-20 20:28 ` Leonardo Bras
2019-09-20 21:15 ` John Hubbard
2019-09-21 0:48 ` John Hubbard
2019-09-23 17:25 ` Leonardo Bras
2019-09-23 18:14 ` John Hubbard [this message]
2019-09-23 19:40 ` Leonardo Bras
2019-09-23 19:58 ` John Hubbard
2019-09-23 20:23 ` Leonardo Bras
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 20:12 ` Leonardo Bras
2019-09-20 21:24 ` John Hubbard
2019-09-23 20:51 ` John Hubbard
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
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