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

[Alejandro Lucero]

On Mon, Oct 29, 2018 at 1:18 PM Yao, Lei A <lei.a.yao at intel.com> wrote:

>
>
>
>
> *From:* Alejandro Lucero [mailto:alejandro.lucero at netronome.com]
> *Sent:* Monday, October 29, 2018 8:56 PM
> *To:* Thomas Monjalon <thomas at monjalon.net>
> *Cc:* Yao, Lei A <lei.a.yao at intel.com>; dev <dev at dpdk.org>; Xu, Qian Q <
> qian.q.xu at intel.com>; Lin, Xueqin <xueqin.lin at intel.com>; Burakov,
> Anatoly <anatoly.burakov at intel.com>; Yigit, Ferruh <ferruh.yigit at intel.com
> >
> *Subject:* Re: [dpdk-dev] [PATCH v3 0/6] use IOVAs check based on DMA mask
>
>
>
>
>
> On Mon, Oct 29, 2018 at 11:46 AM Thomas Monjalon <thomas at 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.

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.


> BRs
>
> Lei
>
>
>
> > 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
>
>