From: Andrew Rybchenko <arybchenko@solarflare.com>
To: Stephen Hemminger <stephen@networkplumber.org>
Cc: <dev@dpdk.org>, Stephen Hemminger <sthemmin@microsoft.com>
Subject: Re: [dpdk-dev] [RFC 01/14] ethdev: add link status read/write functions
Date: Mon, 17 Jul 2017 19:31:30 +0300 [thread overview]
Message-ID: <96c1a91b-13f9-1aa8-a8a4-eb9be98b8751@solarflare.com> (raw)
In-Reply-To: <20170717092151.507a9ecb@xeon-e3>
On 07/17/2017 07:21 PM, Stephen Hemminger wrote:
> On Mon, 17 Jul 2017 19:12:01 +0300
> Andrew Rybchenko <arybchenko@solarflare.com> wrote:
>
>> On 07/17/2017 06:58 PM, Stephen Hemminger wrote:
>>> On Sun, 16 Jul 2017 16:26:06 +0300
>>> Andrew Rybchenko <arybchenko@solarflare.com> wrote:
>>>
>>>> On 07/14/2017 09:30 PM, Stephen Hemminger wrote:
>>>>> Many drivers are all doing copy/paste of the same code to atomicly
>>>>> update the link status. Reduce duplication, and allow for future
>>>>> changes by having common function for this.
>>>>>
>>>>> Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
>>>>> ---
>>>>> lib/librte_ether/rte_ethdev.c | 36 ++++++++++++++++++++++++++++++++++++
>>>>> lib/librte_ether/rte_ethdev.h | 28 ++++++++++++++++++++++++++++
>>>>> 2 files changed, 64 insertions(+)
>>>>>
>>>>> diff --git a/lib/librte_ether/rte_ethdev.c b/lib/librte_ether/rte_ethdev.c
>>>>> index a1b744704f3a..7532fc6b65f0 100644
>>>>> --- a/lib/librte_ether/rte_ethdev.c
>>>>> +++ b/lib/librte_ether/rte_ethdev.c
>>>>> @@ -1332,6 +1332,42 @@ rte_eth_link_get_nowait(uint8_t port_id, struct rte_eth_link *eth_link)
>>>>> }
>>>>>
>>>>> int
>>>>> +_rte_eth_link_update(struct rte_eth_dev *dev,
>>>>> + const struct rte_eth_link *link)
>>>>> +{
>>>>> + volatile struct rte_eth_link *dev_link = &(dev->data->dev_link);
>>>>> + struct rte_eth_link old;
>>>>> +
>>>>> + RTE_BUILD_BUG_ON(sizeof(*link) != sizeof(uint64_t));
>>>>> +
>>>>> + old = *dev_link;
>>>>> +
>>>>> + /* Only reason we use cmpset rather than set is
>>>>> + * that on some architecture may use sign bit as a flag value.
>>>> May I ask to provide more details here.
>>> rte_atomic64_set() takes an int64 argument.
>>> This code (taken from ixgbe, virtio and other drivers) uses cmpset
>>> to allow using uint64_t.
>>>
>>> My assumption is that some architecture in the past was using the
>>> sign bit a a lock value or something. On 64 bit no special support
>>> for 64bit atomic assignment is necessary. Not sure how this code
>>> got inherited that way.
>> Many thanks. May be it would be useful in the comment as well.
> Maybe one of the original developers could clarify.
> It would be cleaner just to do rte_atomcic64_set(), it might just
> be a leftover semantic from Linux/BSD/??? where the original developer
> was looking.
Agree.
>>>>> + */
>>>>> + while (rte_atomic64_cmpset((volatile uint64_t *)dev_link,
>>>>> + *(volatile uint64_t *)dev_link,
>>>>> + *(const uint64_t *)link) == 0)
>>>> Shouldn't it be:
>>>> do {
>>>> old = *dev_link;
>>>> } while (rte_atomic64_cmpset((volatile uint64_t *)dev_link,
>>>> *(uint64_t *)&old, *(const uint64_t *)link) == 0);
>>>>
>>>> At least it has some sense to guarantee transition from old to new
>>>> talking below comparison into account.
>>> Since dev_link is volatile, the compiler is required to refetch
>>> the pointer every time it evaluates the expression. Maybe clearer
>>> to alias devlink to a volatile uint64_t ptr.
>> I meant that dev_link value may change after old value saved in original
>> patch,
>> but before cmpset which actually replaces dev_link value here. As the result
>> two _rte_eth_link_update() run in parallel changing to the same value
>> may return
>> "changes done", but actually only one did the job.
>> I'm not sure if it is really important here, since requirements are not
>> clear.
> Since there is no locking here. There can not be a guarantee of ordering possible.
> The only guarantee is that the set of values (duplex, speed, flags) is consistent.
> I.e one caller wins, the streams don't get crossed.
Results of the update operation is used by some driver to log link up/down
change. So, it could result in duplicate up/down logs. Not a big deal, but
could be confusing.
I guess many are very busy right now with 17.08 release. So, I hope
we'll see more feedback when 17.08 release is done.
next prev parent reply other threads:[~2017-07-17 16:31 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-14 18:30 [dpdk-dev] [RFC 00/14] link status API improvement and bugfixes Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 01/14] ethdev: add link status read/write functions Stephen Hemminger
2017-07-16 13:26 ` Andrew Rybchenko
2017-07-17 15:58 ` Stephen Hemminger
2017-07-17 16:12 ` Andrew Rybchenko
2017-07-17 16:21 ` Stephen Hemminger
2017-07-17 16:31 ` Andrew Rybchenko [this message]
2017-10-11 8:32 ` Yang, Qiming
2017-10-13 15:12 ` Stephen Hemminger
2018-01-05 14:24 ` Thomas Monjalon
2018-01-05 20:15 ` Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 02/14] virtio: use eth_link_read/write (and bug fix) Stephen Hemminger
2017-07-16 12:33 ` Andrew Rybchenko
2017-07-17 16:01 ` Stephen Hemminger
2017-07-17 16:14 ` [dpdk-dev] ***Spam*** " Andrew Rybchenko
2017-07-17 16:28 ` [dpdk-dev] " Stephen Hemminger
2018-01-05 15:04 ` Thomas Monjalon
2017-07-14 18:30 ` [dpdk-dev] [RFC 03/14] bnxt: use rte_link_update Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 04/14] vmxnet3: use rte_eth_link_update Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 05/14] dpaa2: " Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 06/14] nfp: " Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 07/14] e1000: " Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 08/14] ixgbe: " Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 09/14] sfc: use new rte_eth_link helpers Stephen Hemminger
2017-07-16 13:48 ` Andrew Rybchenko
2017-07-17 16:02 ` Stephen Hemminger
2017-07-17 16:19 ` Andrew Rybchenko
2017-07-14 18:30 ` [dpdk-dev] [RFC 10/14] i40e: use rte_eth_link_update (and bug fix) Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 11/14] liquidio: use _rte_eth_link_update Stephen Hemminger
2017-07-18 10:17 ` Shijith Thotton
2017-07-14 18:30 ` [dpdk-dev] [RFC 12/14] thunderx: " Stephen Hemminger
2017-07-14 18:30 ` [dpdk-dev] [RFC 13/14] szedata: " Stephen Hemminger
2017-07-16 12:46 ` Andrew Rybchenko
2017-07-14 18:30 ` [dpdk-dev] [RFC 14/14] enic: " Stephen Hemminger
2017-07-16 13:55 ` [dpdk-dev] [RFC 00/14] link status API improvement and bugfixes Andrew Rybchenko
2018-01-05 14:29 ` Thomas Monjalon
2018-01-05 20:18 ` Stephen Hemminger
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