Re: [PATCH 1/1] migration/multifd: fix channel count TOCTOU race on cancel and retry

[Peter Xu <peterx@redhat.com>]

On Thu, Apr 23, 2026 at 03:13:42PM -0300, Fabiano Rosas wrote:
> Looking again at this argument I put (too many variables), I notice we
> also have multifd_send_state->channels_ready and

channels_ready seems to be special?  In busy systems I think it should
normally always be less than the number of threads on sender side, because
some of them will be busy.

> multifd_recv_state->count, both of which should contain the right number

This one is used so far to just count how many channels are established on
dest side.  Logically after setup phase it can be gone.  It's just not hurt
to be available until cleanups.

> of channels after multifd_send_setup() and migration_start_incoming(),
> respectively. Maybe we could unify all of this into a single semaphore
> used in both sides and take the semaphore count as number of
> channels. @Peter, do you think it's worth it?

My memory is reading counter from a sem isn't always reliable.  I always
only use sem as thread-safety purpose primitives.

So if I was correct on channels_ready above, then it may not apply on the
idea to merge.  We're essentially trying to save one 8B for ->count.. and
it can only be gone after it's finished its use (IOW, we still need it to
work before recv side setup).

Just leave them as-is?

-- 
Peter Xu