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From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: mattias.ronnblom <mattias.ronnblom@ericsson.com>,
	Ola Liljedahl <ola.liljedahl@arm.com>,
	"dev@dpdk.org" <dev@dpdk.org>
Cc: Thomas Monjalon <thomas@monjalon.net>,
	David Marchand <david.marchand@redhat.com>,
	"Olsen, Onar" <onar.olsen@ericsson.com>,
	"Honnappa.Nagarahalli@arm.com" <Honnappa.Nagarahalli@arm.com>,
	"nd@arm.com" <nd@arm.com>,
	"mb@smartsharesystems.com" <mb@smartsharesystems.com>,
	"stephen@networkplumber.org" <stephen@networkplumber.org>
Subject: RE: [RFC] eal: add seqlock
Date: Tue, 29 Mar 2022 13:20:33 +0000	[thread overview]
Message-ID: <DM6PR11MB4491B6E0429B88E5BD077DEF9A1E9@DM6PR11MB4491.namprd11.prod.outlook.com> (raw)
In-Reply-To: <c93ccfe3-9647-68df-af31-fa648f88db3a@ericsson.com>


> >>>>> diff --git a/lib/eal/include/meson.build
> >>>>> b/lib/eal/include/meson.build
> >>>>> index 9700494816..48df5f1a21 100644
> >>>>> --- a/lib/eal/include/meson.build
> >>>>> +++ b/lib/eal/include/meson.build
> >>>>> @@ -36,6 +36,7 @@ headers += files(
> >>>>>            'rte_per_lcore.h',
> >>>>>            'rte_random.h',
> >>>>>            'rte_reciprocal.h',
> >>>>> +        'rte_seqlock.h',
> >>>>>            'rte_service.h',
> >>>>>            'rte_service_component.h',
> >>>>>            'rte_string_fns.h',
> >>>>> diff --git a/lib/eal/include/rte_seqlock.h
> >>>>> b/lib/eal/include/rte_seqlock.h
> >>>>> new file mode 100644
> >>>>> index 0000000000..b975ca848a
> >>>>> --- /dev/null
> >>>>> +++ b/lib/eal/include/rte_seqlock.h
> >>>>> @@ -0,0 +1,84 @@
> >>>>> +/* SPDX-License-Identifier: BSD-3-Clause
> >>>>> + * Copyright(c) 2022 Ericsson AB
> >>>>> + */
> >>>>> +
> >>>>> +#ifndef _RTE_SEQLOCK_H_
> >>>>> +#define _RTE_SEQLOCK_H_
> >>>>> +
> >>>>> +#include <stdbool.h>
> >>>>> +#include <stdint.h>
> >>>>> +
> >>>>> +#include <rte_atomic.h>
> >>>>> +#include <rte_branch_prediction.h>
> >>>>> +#include <rte_spinlock.h>
> >>>>> +
> >>>>> +struct rte_seqlock {
> >>>>> +    uint64_t sn;
> >>>>> +    rte_spinlock_t lock;
> >>>>> +};
> >>>>> +
> >>>>> +typedef struct rte_seqlock rte_seqlock_t;
> >>>>> +
> >>>>> +__rte_experimental
> >>>>> +void
> >>>>> +rte_seqlock_init(rte_seqlock_t *seqlock);
> >>>> Probably worth to have static initializer too.
> >>>>
> >>>
> >>> I will add that in the next version, thanks.
> >>>
> >>>>> +
> >>>>> +__rte_experimental
> >>>>> +static inline uint64_t
> >>>>> +rte_seqlock_read_begin(const rte_seqlock_t *seqlock)
> >>>>> +{
> >>>>> +    /* __ATOMIC_ACQUIRE to prevent loads after (in program order)
> >>>>> +     * from happening before the sn load. Syncronizes-with the
> >>>>> +     * store release in rte_seqlock_end().
> >>>>> +     */
> >>>>> +    return __atomic_load_n(&seqlock->sn, __ATOMIC_ACQUIRE);
> >>>>> +}
> >>>>> +
> >>>>> +__rte_experimental
> >>>>> +static inline bool
> >>>>> +rte_seqlock_read_retry(const rte_seqlock_t *seqlock, uint64_t
> >>>>> begin_sn)
> >>>>> +{
> >>>>> +    uint64_t end_sn;
> >>>>> +
> >>>>> +    /* make sure the data loads happens before the sn load */
> >>>>> +    rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> >>>> That's sort of 'read_end' correct?
> >>>> If so, shouldn't it be '__ATOMIC_RELEASE' instead here,
> >>>> and
> >>>> end_sn = __atomic_load_n(..., (__ATOMIC_ACQUIRE)
> >>>> on the line below?
> >>>
> >>> A release fence prevents reordering of stores. The reader doesn't do
> >>> any
> >>> stores, so I don't understand why you would use a release fence here.
> >>> Could you elaborate?
> >>
> >>  From my understanding:
> >> rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> >> serves as a hoist barrier here, so it would only prevent later
> >> instructions
> >> to be executed before that point.
> >> But it wouldn't prevent earlier instructions to be executed after
> >> that point.
> >> While we do need to guarantee that cpu will finish all previous reads
> >> before
> >> progressing further.
> >>
> >> Suppose we have something like that:
> >>
> >> struct {
> >>     uint64_t shared;
> >>     rte_seqlock_t lock;
> >> } data;
> >>
> >> ...
> >> sn = ...
> >> uint64_t x = data.shared;
> >> /* inside rte_seqlock_read_retry(): */
> >> ...
> >> rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> >> end_sn = __atomic_load_n(&data.lock.sn, __ATOMIC_RELAXED);
> >>
> >> Here we need to make sure that read of data.shared will always happen
> >> before reading of data.lock.sn.
> >> It is not a problem on IA (as reads are not reordered), but on
> >> machines with
> >> relaxed memory ordering (ARM, etc.)  it can happen.
> >> So to prevent it we do need a sink barrier here first (ATOMIC_RELEASE)
> > We can't use store-release since there is no write on the reader-side.
> > And fence-release orders against later stores, not later loads.
> >
> >>
> >> Honnappa and other ARM & atomics experts, please correct me if I am
> >> wrong here.
> > The C standard (chapter 7.17.4 in the C11 (draft)) isn't so easy to
> > digest. If we trust Preshing, he has a more accessible description
> > here:
> > https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-f4f5b1eec2980283&q=1&e=3479ebfa-e18d-4bf8-
> 88fe-76823a531912&u=https%3A%2F%2Fpreshing.com%2F20130922%2Facquire-and-release-fences%2F
> > "An acquire fence prevents the memory reordering of any read which
> > precedes it in program order with any read or write which follows it
> > in program order."
> > and here:
> > https://protect2.fireeye.com/v1/url?k=31323334-501d5122-313273af-454445555731-64b0eba450be934b&q=1&e=3479ebfa-e18d-4bf8-
> 88fe-76823a531912&u=https%3A%2F%2Fpreshing.com%2F20131125%2Facquire-and-release-fences-dont-work-the-way-youd-expect%2F
> > (for C++ but the definition seems to be identical to that of C11).
> > Essentially a LoadLoad+LoadStore barrier which is what we want to
> > achieve.
> >
> > GCC 10.3 for AArch64/A64 ISA generates a "DMB ISHLD" instruction. This
> > waits for all loads preceding (in program order) the memory barrier to
> > be observed before any memory accesses after (in program order) the
> > memory barrier.
> >
> > I think the key to understanding atomic thread fences is that they are
> > not associated with a specific memory access (unlike load-acquire and
> > store-release) so they can't order earlier or later memory accesses
> > against some specific memory access. Instead the fence orders any/all
> > earlier loads and/or stores against any/all later loads or stores
> > (depending on acquire or release).
> >
> >>
> >>>>> +
> >>>>> +    end_sn = __atomic_load_n(&seqlock->sn, __ATOMIC_RELAXED);
> >>>>> +
> >>>>> +    return unlikely(begin_sn & 1 || begin_sn != end_sn);
> >>>>> +}
> >>>>> +
> >>>>> +__rte_experimental
> >>>>> +static inline void
> >>>>> +rte_seqlock_write_begin(rte_seqlock_t *seqlock)
> >>>>> +{
> >>>>> +    uint64_t sn;
> >>>>> +
> >>>>> +    /* to synchronize with other writers */
> >>>>> +    rte_spinlock_lock(&seqlock->lock);
> >>>>> +
> >>>>> +    sn = seqlock->sn + 1;
> >>>>> +
> >>>>> +    __atomic_store_n(&seqlock->sn, sn, __ATOMIC_RELAXED);
> >>>>> +
> >>>>> +    /* __ATOMIC_RELEASE to prevent stores after (in program order)
> >>>>> +     * from happening before the sn store.
> >>>>> +     */
> >>>>> +    rte_atomic_thread_fence(__ATOMIC_RELEASE);
> >>>> I think it needs to be '__ATOMIC_ACQUIRE' here instead of
> >>>> '__ATOMIC_RELEASE'.
> >>>
> >>> Please elaborate on why.
> >>
> >> As you said in the comments above, we need to prevent later stores
> >> to be executed before that point. So we do need a hoist barrier here.
> >> AFAIK to guarantee a hoist barrier '__ATOMIC_ACQUIRE' is required.
> > An acquire fence wouldn't order an earlier store (the write to
> > seqlock->sn) from being reordered with some later store (e.g. writes
> > to the protected data), thus it would allow readers to see updated
> > data (possibly torn) with a pre-update sequence number. We need a
> > StoreStore barrier for ordering the SN store and data stores =>
> > fence(release).
> >
> > Acquire and releases fences can (also) be used to create
> > synchronize-with relationships (this is how the C standard defines
> > them). Preshing has a good example on this. Basically
> > Thread 1:
> > data = 242;
> > atomic_thread_fence(atomic_release);
> > atomic_store_n(&guard, 1, atomic_relaxed);
> >
> > Thread 2:
> > while (atomic_load_n(&guard, atomic_relaxed) != 1) ;
> > atomic_thread_fence(atomic_acquire);
> > do_something(data);
> >
> > These are obvious analogues to store-release and load-acquire, thus
> > the acquire & release names of the fences.
> >
> > - Ola
> >
> >>
> >>>
> >>>>> +}
> >>>>> +
> >>>>> +__rte_experimental
> >>>>> +static inline void
> >>>>> +rte_seqlock_write_end(rte_seqlock_t *seqlock)
> >>>>> +{
> >>>>> +    uint64_t sn;
> >>>>> +
> >>>>> +    sn = seqlock->sn + 1;
> >>>>> +
> >>>>> +    /* synchronizes-with the load acquire in rte_seqlock_begin() */
> >>>>> +    __atomic_store_n(&seqlock->sn, sn, __ATOMIC_RELEASE);
> >>>>> +
> >>>>> +    rte_spinlock_unlock(&seqlock->lock);
> >>>>> +}
> >>>>> +
> >>
> 
> I have nothing to add, but Ola's mail seems to have been blocked from
> the dev list, so I'm posting this again.

Ok, thanks Ola for detailed explanation.
Have to admit then that my understanding of atomic_fence() behaviour was incorrect.
Please disregard my comments above about rte_seqlock_read_retry()
and  rte_seqlock_write_begin().

Konstantin  



  reply	other threads:[~2022-03-29 13:20 UTC|newest]

Thread overview: 104+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-22 16:10 DPDK seqlock Mattias Rönnblom
2022-03-22 16:46 ` Ananyev, Konstantin
2022-03-24  4:52   ` Honnappa Nagarahalli
2022-03-24  5:06     ` Stephen Hemminger
2022-03-24 11:34     ` Mattias Rönnblom
2022-03-25 20:24       ` [RFC] eal: add seqlock Mattias Rönnblom
2022-03-25 21:10         ` Stephen Hemminger
2022-03-26 14:57           ` Mattias Rönnblom
2022-03-27 14:49         ` Ananyev, Konstantin
2022-03-27 17:42           ` Mattias Rönnblom
2022-03-28 10:53             ` Ananyev, Konstantin
2022-03-28 14:06               ` Ola Liljedahl
2022-03-29  8:32                 ` Mattias Rönnblom
2022-03-29 13:20                   ` Ananyev, Konstantin [this message]
2022-03-30 10:07                     ` [PATCH] " Mattias Rönnblom
2022-03-30 10:50                       ` Morten Brørup
2022-03-30 11:24                         ` Tyler Retzlaff
2022-03-30 11:25                         ` Mattias Rönnblom
2022-03-30 14:26                         ` [PATCH v2] " Mattias Rönnblom
2022-03-31  7:46                           ` Mattias Rönnblom
2022-03-31  9:04                             ` Ola Liljedahl
2022-03-31  9:25                               ` Morten Brørup
2022-03-31  9:38                                 ` Ola Liljedahl
2022-03-31 10:03                                   ` Morten Brørup
2022-03-31 11:44                                     ` Ola Liljedahl
2022-03-31 11:50                                       ` Morten Brørup
2022-03-31 14:02                                       ` Mattias Rönnblom
2022-04-01 15:07                                         ` [PATCH v3] " Mattias Rönnblom
2022-04-02  0:21                                           ` Honnappa Nagarahalli
2022-04-02 11:01                                             ` Morten Brørup
2022-04-02 19:38                                               ` Honnappa Nagarahalli
2022-04-10 13:51                                                 ` [RFC 1/3] eal: add macro to warn for unused function return values Mattias Rönnblom
2022-04-10 13:51                                                   ` [RFC 2/3] eal: emit warning for unused trylock return value Mattias Rönnblom
2022-04-10 13:51                                                   ` [RFC 3/3] examples/bond: fix invalid use of trylock Mattias Rönnblom
2022-04-11  1:01                                                     ` Min Hu (Connor)
2022-04-11 14:32                                                       ` Mattias Rönnblom
2022-04-11 11:25                                                     ` David Marchand
2022-04-11 14:33                                                       ` Mattias Rönnblom
2022-04-10 18:02                                                   ` [RFC 1/3] eal: add macro to warn for unused function return values Stephen Hemminger
2022-04-10 18:50                                                     ` Mattias Rönnblom
2022-04-11  7:17                                                   ` Morten Brørup
2022-04-11 14:29                                                     ` Mattias Rönnblom
2022-04-11  9:16                                                   ` Bruce Richardson
2022-04-11 14:27                                                     ` Mattias Rönnblom
2022-04-11 15:15                                                     ` [PATCH " Mattias Rönnblom
2022-04-11 15:15                                                       ` [PATCH 2/3] eal: emit warning for unused trylock return value Mattias Rönnblom
2022-04-11 15:29                                                         ` Morten Brørup
2022-04-11 15:15                                                       ` [PATCH 3/3] examples/bond: fix invalid use of trylock Mattias Rönnblom
2022-04-14 12:06                                                         ` David Marchand
2022-04-11 15:25                                                       ` [PATCH 1/3] eal: add macro to warn for unused function return values Morten Brørup
2022-04-11 18:24                                                     ` [RFC " Tyler Retzlaff
2022-04-03  6:10                                             ` [PATCH v3] eal: add seqlock Mattias Rönnblom
2022-04-03 17:27                                               ` Honnappa Nagarahalli
2022-04-03 18:37                                                 ` Ola Liljedahl
2022-04-04 21:56                                                   ` Honnappa Nagarahalli
2022-04-03  6:33                                             ` Mattias Rönnblom
2022-04-03 17:37                                               ` Honnappa Nagarahalli
2022-04-08 13:45                                                 ` Mattias Rönnblom
2022-04-02 18:15                                           ` Ola Liljedahl
2022-04-02 19:31                                             ` Honnappa Nagarahalli
2022-04-02 20:36                                               ` Morten Brørup
2022-04-02 22:01                                                 ` Honnappa Nagarahalli
2022-04-03 18:11                                               ` Ola Liljedahl
2022-04-03  6:51                                             ` Mattias Rönnblom
2022-03-31 13:51                                 ` [PATCH v2] " Mattias Rönnblom
2022-04-02  0:54                                   ` Stephen Hemminger
2022-04-02 10:25                                     ` Morten Brørup
2022-04-02 17:43                                       ` Ola Liljedahl
2022-03-31 13:38                               ` Mattias Rönnblom
2022-03-31 14:53                                 ` Ola Liljedahl
2022-04-02  0:52                                   ` Stephen Hemminger
2022-04-03  6:23                                     ` Mattias Rönnblom
2022-04-02  0:50                           ` Stephen Hemminger
2022-04-02 17:54                             ` Ola Liljedahl
2022-04-02 19:37                               ` Honnappa Nagarahalli
2022-04-05 20:16                           ` Stephen Hemminger
2022-04-08 13:50                             ` Mattias Rönnblom
2022-04-08 14:24                               ` [PATCH v4] " Mattias Rönnblom
2022-04-08 15:17                                 ` Stephen Hemminger
2022-04-08 16:24                                   ` Mattias Rönnblom
2022-04-08 15:19                                 ` Stephen Hemminger
2022-04-08 16:37                                   ` Mattias Rönnblom
2022-04-08 16:48                                 ` Mattias Rönnblom
2022-04-12 17:27                                 ` Ananyev, Konstantin
2022-04-28 10:28                                 ` David Marchand
2022-05-01 13:46                                   ` Mattias Rönnblom
2022-05-01 14:03                                     ` [PATCH v5] " Mattias Rönnblom
2022-05-01 14:22                                       ` Mattias Rönnblom
2022-05-02  6:47                                         ` David Marchand
2022-05-01 20:17                                       ` Stephen Hemminger
2022-05-02  4:51                                         ` Mattias Rönnblom
2022-05-06  1:26                                       ` fengchengwen
2022-05-06  1:33                                         ` Honnappa Nagarahalli
2022-05-06  4:17                                           ` fengchengwen
2022-05-06  5:19                                             ` Honnappa Nagarahalli
2022-05-06  7:03                                               ` fengchengwen
2022-05-08 11:56                                         ` Mattias Rönnblom
2022-05-08 12:12                                           ` [PATCH v6] " Mattias Rönnblom
2022-05-08 16:10                                             ` Stephen Hemminger
2022-05-08 19:40                                               ` Mattias Rönnblom
2022-05-09  3:48                                                 ` Stephen Hemminger
2022-05-09  6:26                                                   ` Morten Brørup
2022-05-13  6:27                                                   ` Mattias Rönnblom
2022-03-23 12:04 ` DPDK seqlock Morten Brørup

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