Re: [dpdk-dev] [PATCH v3 0/6] use IOVAs check based on DMA mask

[Alejandro Lucero <alejandro.lucero@netronome.com>]

On Mon, Oct 29, 2018 at 6:54 PM Yongseok Koh <yskoh@mellanox.com> wrote:

>
> > On Oct 29, 2018, at 7:18 AM, Thomas Monjalon <thomas@monjalon.net>
> wrote:
> >
> > 29/10/2018 14:40, Alejandro Lucero:
> >> On Mon, Oct 29, 2018 at 1:18 PM Yao, Lei A <lei.a.yao@intel.com> wrote:
> >>> *From:* Alejandro Lucero [mailto:alejandro.lucero@netronome.com]
> >>> On Mon, Oct 29, 2018 at 11:46 AM Thomas Monjalon <thomas@monjalon.net>
> >>> wrote:
> >>>
> >>> 29/10/2018 12:39, Alejandro Lucero:
> >>>> I got a patch that solves a bug when calling rte_eal_dma_mask using
> the
> >>>> mask instead of the maskbits. However, this does not solves the
> >>> deadlock.
> >>>
> >>> The deadlock is a bigger concern I think.
> >>>
> >>> I think once the call to rte_eal_check_dma_mask uses the maskbits
> instead
> >>> of the mask, calling rte_memseg_walk_thread_unsafe avoids the deadlock.
> >>>
> >>> Yao, can you try with the attached patch?
> >>>
> >>> Hi, Lucero
> >>>
> >>> This patch can fix the issue at my side. Thanks a lot
> >>> for you quick action.
> >>
> >> Great!
> >>
> >> I will send an official patch with the changes.
> >
> > Please, do not forget my other request to better comment functions.
>
> Alejandro,
>
> This patchset has been merged to stable/17.11 per your request for the
> last release.
> You must send a fix to stable/17.11 as well, if you think there's a same
> issue there.
>
>
The patchset for 17.11 was much more simpler. There have been a lot of
changes to the memory code since 17.11, and this problem should not be
present in stable 17.11.

Once I have said that, if there are any reports about a problem with this
patchset in 17.11, I will work on it as a priority.

Thanks.


> Thanks,
> Yongseok
>
> >> I have to say that I tested the patchset, but I think it was where
> >> legacy_mem was still there and therefore dynamic memory allocation code
> not
> >> used during memory initialization.
> >>
> >> There is something that concerns me though. Using
> >> rte_memseg_walk_thread_unsafe could be a problem under some situations
> >> although those situations being unlikely.
> >>
> >> Usually, calling rte_eal_check_dma_mask happens during initialization.
> Then
> >> it is safe to use the unsafe function for walking memsegs, but with
> device
> >> hotplug and dynamic memory allocation, there exists a potential race
> >> condition when the primary process is allocating more memory and
> >> concurrently a device is hotplugged and a secondary process does the
> device
> >> initialization. By now, this is just a problem with the NFP, and the
> >> potential race condition window really unlikely, but I will work on this
> >> asap.
> >
> > Yes, this is what concerns me.
> > You can add a comment explaining the unsafe which is not handled.
> >
> >
> >>>> Interestingly, the problem looks like a compiler one. Calling
> >>>> rte_memseg_walk does not return when calling inside rt_eal_dma_mask,
> >>> but if
> >>>> you modify the call like this:
> >>>>
> >>>> -       if (rte_memseg_walk(check_iova, &mask))
> >>>> +       if (!rte_memseg_walk(check_iova, &mask))
> >>>>
> >>>> it works, although the value returned to the invoker changes, of
> course.
> >>>> But the point here is it should be the same behaviour when calling
> >>>> rte_memseg_walk than before and it is not.
> >>>
> >>> Anyway, the coding style requires to save the return value in a
> variable,
> >>> instead of nesting the call in an "if" condition.
> >>> And the "if" check should be explicitly != 0 because it is not a real
> >>> boolean.
> >>>
> >>> PS: please do not top post and avoid HTML emails, thanks
> >>>
> >>>
> >>
> >
> >
> >
> >
> >
>
>