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

[Thomas Monjalon <thomas@monjalon.net>]

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.


> 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
> >
> >
>