migration/multifd: Optimize sender side to be lockless

When reviewing my attempt to refactor send_prepare(), Fabiano suggested we
try out with dropping the mutex in multifd code [1].

I thought about that before but I never tried to change the code.  Now
maybe it's time to give it a stab.  This only optimizes the sender side.

The trick here is multifd has a clear provider/consumer model, that the
migration main thread publishes requests (either pending_job/pending_sync),
while the multifd sender threads are consumers.  Here we don't have a lot
of complicated data sharing, and the jobs can logically be submitted
lockless.

Arm the code with atomic weapons.  Two things worth mentioning:

  - For multifd_send_pages(): we can use qatomic_load_acquire() when trying
  to find a free channel, but that's expensive if we attach one ACQUIRE per
  channel.  Instead, keep the qatomic_read() on reading the pending_job
  flag as we do already, meanwhile use one smp_mb_acquire() after the loop
  to guarantee the memory ordering.

  - For pending_sync: it doesn't have any extra data required since now
  p->flags are never touched, it should be safe to not use memory barrier.
  That's different from pending_job.

Provide rich comments for all the lockless operations to state how they are
paired.  With that, we can remove the mutex.

[1] https://lore.kernel.org/r/87o7d1jlu5.fsf@suse.de

Suggested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240202102857.110210-24-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/multifd.h

@@ -91,8 +91,6 @@ typedef struct {
     /* syncs main thread and channels */
     QemuSemaphore sem_sync;
 
-    /* this mutex protects the following parameters */
-    QemuMutex mutex;
     /* is this channel thread running */
     bool running;
     /* multifd flags for each packet */