RE: [PATCH 2/2] ethdev: fix the race condition for fp ops reset

[Bing Zhao <bingz@nvidia.com>]

Yes +1

Let the application handle it once there is an issue.

BR. Bing

> -----Original Message-----
> From: Thomas Monjalon <thomas@monjalon.net>
> Sent: Wednesday, November 10, 2021 10:58 PM
> To: Yigit, Ferruh <ferruh.yigit@intel.com>; Bing Zhao
> <bingz@nvidia.com>; Ananyev, Konstantin
> <konstantin.ananyev@intel.com>
> Cc: andrew.rybchenko@oktetlabs.ru; dev@dpdk.org
> Subject: Re: [PATCH 2/2] ethdev: fix the race condition for fp ops
> reset
> 
> External email: Use caution opening links or attachments
> 
> 
> 10/11/2021 15:37, Ananyev, Konstantin:
> >
> > 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.
> 
> Yes
>