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: 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: 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 >
WARNING: multiple messages have this Message-ID (diff)
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 18:14 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 [this message] 2019-09-23 18:14 ` John Hubbard 2019-09-23 19:40 ` Leonardo Bras 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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