Re: [PATCH v2 22/23] migration/multifd: Fix MultiFDSendParams.packet_num race

[Peter Xu <peterx@redhat.com>]

On Fri, Feb 02, 2024 at 06:08:22PM -0300, Fabiano Rosas wrote:
> peterx@redhat.com writes:
> 
> > From: Peter Xu <peterx@redhat.com>
> >
> > As reported correctly by Fabiano [1], MultiFDSendParams.packet_num is buggy
> > to be assigned and stored.  Consider two consequent operations of: (1)
> > queue a job into multifd send thread X, then (2) queue another sync request
> > to the same send thread X.  Then the MultiFDSendParams.packet_num will be
> > assigned twice, and the first assignment can get lost already.
> >
> > To avoid that, we move the packet_num assignment from p->packet_num into
> > where the thread will fill in the packet.  Use atomic operations to protect
> > the field, making sure there's no race.
> >
> > Note that atomic fetch_add() may not be good for scaling purposes, however
> > multifd should be fine as number of threads should normally not go beyond
> > 16 threads.  Let's leave that concern for later but fix the issue first.
> >
> > There's also a trick on how to make it always work even on 32 bit hosts for
> > uint64_t packet number.  Switching to uintptr_t as of now to simply the
> > case.  It will cause packet number to overflow easier on 32 bit, but that
> > shouldn't be a major concern for now as 32 bit systems is not the major
> > audience for any performance concerns like what multifd wants to address.
> >
> > We also need to move multifd_send_state definition upper, so that
> > multifd_send_fill_packet() can reference it.
> >
> > [1] https://lore.kernel.org/r/87o7d1jlu5.fsf@suse.de
> >
> > Reported-by: Fabiano Rosas <farosas@suse.de>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> Elena had reported this in October already.
> 
> Reported-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>

Ah, I'll do the replacement.

> Reviewed-by: Fabiano Rosas <farosas@suse.de>

Thanks,

-- 
Peter Xu