From: "Mattias Rönnblom" <mattias.ronnblom@ericsson.com>
To: Ola Liljedahl <ola.liljedahl@arm.com>,
"Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
"dev@dpdk.org" <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" <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 08:32:20 +0000 [thread overview]
Message-ID: <c93ccfe3-9647-68df-af31-fa648f88db3a@ericsson.com> (raw)
In-Reply-To: <e730db91-d1e7-3ced-c29e-6ceb7bb629b0@arm.com>
On 2022-03-28 16:06, Ola Liljedahl wrote:
>
>
> On 3/28/22 12:53, Ananyev, Konstantin wrote:
>>
>>>>> 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.
next prev parent reply other threads:[~2022-03-29 8:32 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 [this message]
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
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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