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
next prev 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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