From: "Mattias Rönnblom" <hofors@lysator.liu.se>
To: Ola Liljedahl <ola.liljedahl@arm.com>, dev@dpdk.org
Cc: Thomas Monjalon <thomas@monjalon.net>,
David Marchand <david.marchand@redhat.com>,
onar.olsen@ericsson.com, Honnappa.Nagarahalli@arm.com,
nd@arm.com, konstantin.ananyev@intel.com,
mb@smartsharesystems.com, stephen@networkplumber.org
Subject: Re: [PATCH v3] eal: add seqlock
Date: Sun, 3 Apr 2022 08:51:39 +0200 [thread overview]
Message-ID: <17b2cdec-a56f-9069-c3ec-13f31fd72f57@lysator.liu.se> (raw)
In-Reply-To: <3efba44b-1a2e-831e-1e2c-782639d83c24@arm.com>
On 2022-04-02 20:15, Ola Liljedahl wrote:
> On 4/1/22 17:07, Mattias Rönnblom wrote:
>> +
>> +/**
>> + * End a read-side critical section.
>> + *
>> + * A call to this function marks the end of a read-side critical
>> + * section, for @p seqlock. The application must supply the sequence
>> + * number produced by the corresponding rte_seqlock_read_lock() (or,
>> + * in case of a retry, the rte_seqlock_tryunlock()) call.
>> + *
>> + * After this function has been called, the caller should not access
>> + * the protected data.
>> + *
>> + * In case this function returns true, the just-read data was
>> + * consistent and the set of atomic and non-atomic load operations
>> + * performed between rte_seqlock_read_lock() and
>> + * rte_seqlock_read_tryunlock() were atomic, as a whole.
>> + *
>> + * In case rte_seqlock_read_tryunlock() returns false, the data was
>> + * modified as it was being read and may be inconsistent, and thus
>> + * should be discarded. The @p begin_sn is updated with the
>> + * now-current sequence number.
>> + *
>> + * @param seqlock
>> + * A pointer to the seqlock.
>> + * @param begin_sn
>> + * The seqlock sequence number returned by
>> + * rte_seqlock_read_lock() (potentially updated in subsequent
>> + * rte_seqlock_read_tryunlock() calls) for this critical section.
>> + * @return
>> + * true or false, if the just-read seqlock-protected data was
>> consistent
>> + * or inconsistent, respectively, at the time it was read.
>> + *
>> + * @see rte_seqlock_read_lock()
>> + */
>> +__rte_experimental
>> +static inline bool
>> +rte_seqlock_read_tryunlock(const rte_seqlock_t *seqlock, uint32_t
>> *begin_sn)
>> +{
>> + uint32_t end_sn;
>> +
>> + /* make sure the data loads happens before the sn load */
>> + rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
>> +
>> + end_sn = __atomic_load_n(&seqlock->sn, __ATOMIC_RELAXED);
>
> Since we are reading and potentially returning the sequence number here
> (repeating the read of the protected data), we need to use load-acquire.
> I assume it is not expected that the user will call
> rte_seqlock_read_lock() again.
>
Good point.
> Seeing this implementation, I might actually prefer the original
> implementation, I think it is cleaner.
Me too.
> But I would like for the begin
> function also to wait for an even sequence number, the end function
> would only have to check for same sequence number, this might improve
> performance a little bit as readers won't perform one or several broken
> reads while a write is in progress. The function names are a different
> thing though.
>
The odd sn should be a rare case, if the seqlock is used for relatively
low frequency update scenarios, which is what I think it should be
designed for.
Waiting for an even sn in read_begin() would exclude the option for the
caller to defer reading the new data to same later time, in case it's
being written. That in turn would force even a single writer to make
sure its thread is not preempted, or risk blocking all lcore worker
cores attempting to read the protected data.
You could complete the above API with a read_trybegin() function to
address that issue, for those who care, but that would force some extra
complexity on the user.
> The writer side behaves much more like a lock with mutual exclusion so
> write_lock/write_unlock makes sense.
>
>> +
>> + if (unlikely(end_sn & 1 || *begin_sn != end_sn)) {
>> + *begin_sn = end_sn;
>> + return false;
>> + }
>> +
>> + return true;
>> +}
>> +
next prev parent reply other threads:[~2022-04-03 6:51 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 [this message]
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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