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From: Ola Liljedahl <ola.liljedahl@arm.com>
To: "Morten Brørup" <mb@smartsharesystems.com>,
	"Mattias Rönnblom" <mattias.ronnblom@ericsson.com>,
	dev@dpdk.org
Cc: Thomas Monjalon <thomas@monjalon.net>,
	David Marchand <david.marchand@redhat.com>,
	Onar Olsen <onar.olsen@ericsson.com>,
	Honnappa.Nagarahalli@arm.com, nd@arm.com,
	konstantin.ananyev@intel.com, stephen@networkplumber.org
Subject: Re: [PATCH v2] eal: add seqlock
Date: Thu, 31 Mar 2022 11:38:54 +0200	[thread overview]
Message-ID: <a44ce729-f877-8b81-a21c-5a9f70ed82e5@arm.com> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35D86F8D@smartserver.smartshare.dk>



On 3/31/22 11:25, Morten Brørup wrote:
>> From: Ola Liljedahl [mailto:ola.liljedahl@arm.com]
>> Sent: Thursday, 31 March 2022 11.05
>>
>> On 3/31/22 09:46, Mattias Rönnblom wrote:
>>> On 2022-03-30 16:26, Mattias Rönnblom wrote:
>>>> 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 with
>>>> relatively infrequently.
>>>>
>>>>
>>>
>>> <snip>
>>>
>>> Some questions I have:
>>>
>>> Is a variant of the seqlock without the spinlock required? The reason
>> I
>>> left such out was that I thought that in most cases where only a
>> single
>>> writer is used (or serialization is external to the seqlock), the
>>> spinlock overhead is negligible, since updates are relatively
>> infrequent.
> 
> Mattias, when you suggested adding the seqlock, I considered this too, and came to the same conclusion as you.
> 
>> You can combine the spinlock and the sequence number. Odd sequence
>> number means the seqlock is busy. That would replace a non-atomic RMW
>> of
>> the sequence number with an atomic RMW CAS and avoid the spin lock
>> atomic RMW operation. Not sure how much it helps.
>>
>>>
>>> Should the rte_seqlock_read_retry() be called rte_seqlock_read_end(),
>> or
>>> some third alternative? I wanted to make clear it's not just a
>> "release
>>> the lock" function. You could use
>>> the|||__attribute__((warn_unused_result)) annotation to make clear
>> the
>>> return value cannot be ignored, although I'm not sure DPDK ever use
>> that
>>> attribute.
> 
> I strongly support adding __attribute__((warn_unused_result)) to the function. There's a first time for everything, and this attribute is very relevant here!
> 
>> We have to decide how to use the seqlock API from the application
>> perspective.
>> Your current proposal:
>> do {
>>       sn = rte_seqlock_read_begin(&seqlock)
>>       //read protected data
>> } while (rte_seqlock_read_retry(&seqlock, sn));
>>
>> or perhaps
>> sn = rte_seqlock_read_lock(&seqlock);
>> do {
>>       //read protected data
>> } while (!rte_seqlock_read_tryunlock(&seqlock, &sn));
>>
>> Tryunlock should signal to the user that the unlock operation might not
>> succeed and something needs to be repeated.
> 
> Perhaps rename rte_seqlock_read_retry() to rte_seqlock_read_tryend()? As Ola mentions, this also inverses the boolean result value. If you consider this, please check that the resulting assembly output remains efficient.
> 
> I think lock()/unlock() should be avoided in the read operation names, because no lock is taken during read. I like the critical region begin()/end() names.
I was following the naming convention of rte_rwlock. Isn't the seqlock 
just a more scalable implementation of a reader/writer lock?


> 
> Regarding naming, you should also consider renaming rte_seqlock_write_begin/end() to rte_seqlock_write_lock/unlock(), following the naming convention of the other locks. This could prepare for future extensions, such as rte_seqlock_write_trylock(). Just a thought; I don't feel strongly about this.
> 
> Ola, the rte_seqlock_read_lock(&seqlock) must remain inside the loop, because retries can be triggered by a write operation happening between the read_begin() and read_tryend(), and then the new sn must be used by the read operation.
That's why my rte_seqlock_read_tryunlock() function takes the sequence 
number as a parameter passed by reference. Then the sequence number can 
be updated if necessary. I didn't want to force a new call to 
rte_seqlock_read_lock() because there should be a one-to-one match 
between rte_seqlock_read_lock() and a successful call to 
rte_seqlock_read_tryunlock().

- Ola
> 

  reply	other threads:[~2022-03-31  9:39 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 [this message]
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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