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From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: mattias.ronnblom <mattias.ronnblom@ericsson.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>,
	"hofors@lysator.liu.se" <hofors@lysator.liu.se>,
	mattias.ronnblom <mattias.ronnblom@ericsson.com>,
	Ola Liljedahl <ola.liljedahl@arm.com>
Subject: RE: [PATCH v4] eal: add seqlock
Date: Tue, 12 Apr 2022 17:27:18 +0000	[thread overview]
Message-ID: <DM6PR11MB449155E2325472EAA4D0515F9AED9@DM6PR11MB4491.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20220408142442.157192-1-mattias.ronnblom@ericsson.com>



> A sequence lock (seqlock) is synchronization primitive which allows
> for data-race free, low-overhead, high-frequency reads, especially for
> data structures shared across many cores and which are updated
> relatively infrequently.
> 
> A seqlock permits multiple parallel readers. The variant of seqlock
> implemented in this patch supports multiple writers as well. A
> spinlock is used for writer-writer serialization.
> 
> To avoid resource reclamation and other issues, the data protected by
> a seqlock is best off being self-contained (i.e., no pointers [except
> to constant data]).
> 
> One way to think about seqlocks is that they provide means to perform
> atomic operations on data objects larger what the native atomic
> machine instructions allow for.
> 
> DPDK seqlocks are not preemption safe on the writer side. A thread
> preemption affects performance, not correctness.
> 
> A seqlock contains a sequence number, which can be thought of as the
> generation of the data it protects.
> 
> A reader will
>   1. Load the sequence number (sn).
>   2. Load, in arbitrary order, the seqlock-protected data.
>   3. Load the sn again.
>   4. Check if the first and second sn are equal, and even numbered.
>      If they are not, discard the loaded data, and restart from 1.
> 
> The first three steps need to be ordered using suitable memory fences.
> 
> A writer will
>   1. Take the spinlock, to serialize writer access.
>   2. Load the sn.
>   3. Store the original sn + 1 as the new sn.
>   4. Perform load and stores to the seqlock-protected data.
>   5. Store the original sn + 2 as the new sn.
>   6. Release the spinlock.
> 
> Proper memory fencing is required to make sure the first sn store, the
> data stores, and the second sn store appear to the reader in the
> mentioned order.
> 
> The sn loads and stores must be atomic, but the data loads and stores
> need not be.
> 
> The original seqlock design and implementation was done by Stephen
> Hemminger. This is an independent implementation, using C11 atomics.
> 
> For more information on seqlocks, see
> https://en.wikipedia.org/wiki/Seqlock
> 
> PATCH v4:
>   * Reverted to Linux kernel style naming on the read side.
>   * Bail out early from the retry function if an odd sequence
>     number is encountered.
>   * Added experimental warnings in the API documentation.
>   * Static initializer now uses named field initialization.
>   * Various tweaks to API documentation (including the example).
> 
> PATCH v3:
>   * Renamed both read and write-side critical section begin/end functions
>     to better match rwlock naming, per Ola Liljedahl's suggestion.
>   * Added 'extern "C"' guards for C++ compatibility.
>   * Refer to the main lcore as the main lcore, and nothing else.
> 
> PATCH v2:
>   * Skip instead of fail unit test in case too few lcores are available.
>   * Use main lcore for testing, reducing the minimum number of lcores
>     required to run the unit tests to four.
>   * Consistently refer to sn field as the "sequence number" in the
>     documentation.
>   * Fixed spelling mistakes in documentation.
> 
> Updates since RFC:
>   * Added API documentation.
>   * Added link to Wikipedia article in the commit message.
>   * Changed seqlock sequence number field from uint64_t (which was
>     overkill) to uint32_t. The sn type needs to be sufficiently large
>     to assure no reader will read a sn, access the data, and then read
>     the same sn, but the sn has been incremented enough times to have
>     wrapped during the read, and arrived back at the original sn.
>   * Added RTE_SEQLOCK_INITIALIZER macro for static initialization.
>   * Removed the rte_seqlock struct + separate rte_seqlock_t typedef
>     with an anonymous struct typedef:ed to rte_seqlock_t.
> 
> Acked-by: Morten Brørup <mb@smartsharesystems.com>
> Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> ---

Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>

> --
> 2.25.1


  parent reply	other threads:[~2022-04-12 17:28 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
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 [this message]
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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