From: Ferruh Yigit <ferruh.yigit@intel.com>
To: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
Thomas Monjalon <thomas@monjalon.net>,
Bing Zhao <bingz@nvidia.com>
Cc: "andrew.rybchenko@oktetlabs.ru" <andrew.rybchenko@oktetlabs.ru>,
"dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [PATCH 2/2] ethdev: fix the race condition for fp ops reset
Date: Wed, 10 Nov 2021 14:34:52 +0000 [thread overview]
Message-ID: <16525cf6-bf10-e637-3f03-f4d415ff6bb1@intel.com> (raw)
In-Reply-To: <DM6PR11MB4491AAD487BBC6A518B27FA99A819@DM6PR11MB4491.namprd11.prod.outlook.com>
On 10/23/2021 12:39 PM, Ananyev, Konstantin wrote:
>
>
>> 22/10/2021 23:14, Bing Zhao:
>>> In the function "eth_dev_fp_ops_reset", a structure assignment
>>> operation is used to reset one queue's callback functions, etc., but
>>> it is not thread safe.
>>>
>>> The structure assignment is not atomic, a lot of instructions will
>>> be generated. Right now, since not all the fields are needed, the
>>> fields in the "dummy_ops" which is not set explicitly will be 0s
>>> based on the specification and compiler behavior. In order to make
>>> "fpo" has the same content with "dummy_ops", some clearing to 0
>>> operation is needed.
>>>
>>> By checking the object instructions (e.g. with GCC 4.8.5)
>>> 0x0000000000a58317 <+35>: mov %rsi,%rdi
>>> 0x0000000000a5831a <+38>: mov %rdx,%rcx
>>> => 0x0000000000a5831d <+41>: rep stos %rax,%es:(%rdi)
>>> 0x0000000000a58320 <+44>: mov -0x38(%rsp),%rax
>>> 0x0000000000a58325 <+49>: lea -0xe0(%rip),%rdx
>>> // # 0xa5824c <dummy_eth_rx_burst>
>>>
>>> It shows that "rep stos" will clear the "fpo" structure before
>>> assigning new values.
>>>
>>> In the other thread, if some data path Tx / Rx functions are still
>>> running, there is a risk to get 0 instead of the correct dummy
>>> content.
>>> 1. qd = p->rxq.data[queue_id]
>>> 2. (void **)&p->rxq.clbk[queue_id]
>>> "data" and "clbk" may be observed with NULL (0) in other threads.
>>> Even it is temporary, the accessing to a NULL pointer will cause a
>>> crash. Using "memcpy" could get rid of this.
>>>
>>> Fixes: c87d435a4d79 ("ethdev: copy fast-path API into separate structure")
>>> Cc: konstantin.ananyev@intel.com
>>>
>>> Signed-off-by: Bing Zhao <bingz@nvidia.com>
>>> ---
>>> --- a/lib/ethdev/ethdev_private.c
>>> +++ b/lib/ethdev/ethdev_private.c
>>> @@ -206,7 +206,7 @@ eth_dev_fp_ops_reset(struct rte_eth_fp_ops *fpo)
>>> .txq = {.data = dummy_data, .clbk = dummy_data,},
>>> };
>>>
>>> - *fpo = dummy_ops;
>>> + rte_memcpy(fpo, &dummy_ops, sizeof(struct rte_eth_fp_ops));
>>
>> That's not trivial.
>> Please add a comment to briefly explain that memcpy avoids zeroing of a simple assignment.
>>
>
> I think that patch is based on two totally wrong assumptions:
> 1) ethdev data-path and control-path API is MT-safe.
> With current design it is not.
> When calling rx/tx_burst it is caller responsibility to make sure that given port is
> already properly configured and started. Also it is user responsibility to guarantee
> that none other thread doing dev_stop for the same port simultaneously.
> And visa-versa when calling dev_stop(), it is user responsibility to ensure that
> none other thread doing rx/tx_burst for given port simultaneously.
> If your app doesn't follow these principles, then it is a bug that needs to be fixed.
> 2) rte_memcpy() provides some sort of atomicity and it is safe to use it on its own
> in MT environment. That's totally wrong.
> In both cases compiler has total freedom to perform copy in any order it likes
> (let say it can first read whole source data in some temporary buffer (SIMD register),
> and then right it in one go, or it can do the same trick with 'rep stos' as above).
> Moreover CPU itself can reorder instructions.
> So if you need this copy to be atomic you need to use some sort of
> sync primitives along with it (mutex, rwlock, rcu, etc.).
> But as I said above right now ethdev API is not MT-safe, so it is not required.
>
> To summarise - there is no point to mae these changes,
> and patch comment is wrong and misleading.
Can we mark this patch as rejected now?
Patch seems trying to cover a wrong application usage, and it should
be addressed in the application level.
next prev parent reply other threads:[~2021-11-10 14:35 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-22 21:14 [dpdk-dev] [PATCH 1/2] ethdev: fix log level of Tx and Rx dummy functions Bing Zhao
2021-10-22 21:14 ` [dpdk-dev] [PATCH 2/2] ethdev: fix the race condition for fp ops reset Bing Zhao
2021-10-23 8:34 ` Thomas Monjalon
2021-10-23 11:39 ` Ananyev, Konstantin
2021-11-10 14:34 ` Ferruh Yigit [this message]
2021-11-10 14:37 ` Ananyev, Konstantin
2021-11-10 14:57 ` Thomas Monjalon
2021-11-10 15:24 ` Bing Zhao
2021-10-23 16:13 ` [dpdk-dev] " Stephen Hemminger
2021-10-24 5:54 ` Bing Zhao
2021-10-23 8:32 ` [dpdk-dev] [PATCH 1/2] ethdev: fix log level of Tx and Rx dummy functions Thomas Monjalon
2021-10-23 11:46 ` Ananyev, Konstantin
2021-10-23 12:45 ` Bing Zhao
2021-10-24 11:48 ` Ananyev, Konstantin
2021-10-25 9:43 ` Thomas Monjalon
2021-10-25 9:51 ` David Marchand
2021-10-25 12:55 ` Ananyev, Konstantin
2021-10-25 13:27 ` Thomas Monjalon
2021-10-25 13:31 ` David Marchand
2021-10-25 20:29 ` Ananyev, Konstantin
2021-10-25 20:38 ` Thomas Monjalon
2021-10-26 12:38 ` Ananyev, Konstantin
2021-10-26 12:59 ` Thomas Monjalon
2021-10-26 3:18 ` Bing Zhao
2021-10-23 12:12 ` Bing Zhao
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