From: Leonardo Bras <leonardo@linux.ibm.com>
To: John Hubbard <jhubbard@nvidia.com>,
linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Cc: Jason Gunthorpe <jgg@ziepe.ca>,
Thomas Gleixner <tglx@linutronix.de>,
Arnd Bergmann <arnd@arndb.de>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
YueHaibing <yuehaibing@huawei.com>,
Keith Busch <keith.busch@intel.com>,
Nicholas Piggin <npiggin@gmail.com>,
Mike Rapoport <rppt@linux.ibm.com>,
Mahesh Salgaonkar <mahesh@linux.vnet.ibm.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>,
Andrew Morton <akpm@linux-foundation.org>,
Ira Weiny <ira.weiny@intel.com>,
Dan Williams <dan.j.williams@intel.com>,
Allison Randal <allison@lohutok.net>
Subject: Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing
Date: Fri, 20 Sep 2019 17:28:41 -0300 [thread overview]
Message-ID: <24863d8904c6e05e5dd48cab57db4274675ae654.camel@linux.ibm.com> (raw)
In-Reply-To: <1b39eaa7-751d-40bc-d3d7-41aaa15be42a@nvidia.com>
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On Fri, 2019-09-20 at 13:11 -0700, John Hubbard wrote:
> On 9/20/19 12:50 PM, Leonardo Bras wrote:
> > Skips slow part of serialize_against_pte_lookup if there is no running
> > lockless pagetable walk.
> >
> > Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
> > ---
> > arch/powerpc/mm/book3s64/pgtable.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> > index 13239b17a22c..41ca30269fa3 100644
> > --- a/arch/powerpc/mm/book3s64/pgtable.c
> > +++ b/arch/powerpc/mm/book3s64/pgtable.c
> > @@ -95,7 +95,8 @@ static void do_nothing(void *unused)
> > void serialize_against_pte_lookup(struct mm_struct *mm)
> > {
> > smp_mb();
> > - smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
> > + if (running_lockless_pgtbl_walk(mm))
> > + smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
>
> Hi,
>
> If you do this, then you are left without any synchronization. So it will
> have race conditions: a page table walk could begin right after the above
> check returns "false", and then code such as hash__pmdp_huge_get_and_clear()
> will continue on right away, under the false assumption that it has let
> all the current page table walks complete.
>
> The current code uses either interrupts or RCU to synchronize, and in
> either case, you end up scheduling something on each CPU. If you remove
> that entirely, I don't see anything left. ("Pure" atomic counting is not
> a synchronization technique all by itself.)
>
> thanks,
Hello John,
Thanks for the fast feedback.
See, before calling serialize_against_pte_lookup(), there is always an
update or clear on the pmd. So, if a page table walk begin right after
the check returns "false", there is no problem, since it will use the
updated pmd.
Think about serialize, on a process with a bunch of cpus. After you
check the last processor (wait part), there is no guarantee that the
first one is not starting a lockless pagetable walk.
The same mechanism protect both methods.
Does it make sense?
Best regards,
Leonardo Bras
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next prev parent reply other threads:[~2019-09-20 20:35 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 [this message]
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
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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