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From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: "Yigit, Ferruh" <ferruh.yigit@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 14:37:33 +0000	[thread overview]
Message-ID: <DM6PR11MB4491FF40BA94F671F7792D169A939@DM6PR11MB4491.namprd11.prod.outlook.com> (raw)
In-Reply-To: <16525cf6-bf10-e637-3f03-f4d415ff6bb1@intel.com>


Hi Ferruh,

> >> 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?

I believe so.

> Patch seems trying to cover a wrong application usage, and it should
> be addressed in the application level.

  reply	other threads:[~2021-11-10 14:37 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
2021-11-10 14:37         ` Ananyev, Konstantin [this message]
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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