[RFC PATCH 1/3] migration/multifd: Move channels_ready semaphore

[Fabiano Rosas <farosas@suse.de>]

Commit d2026ee117 ("multifd: Fix the number of channels ready") moved
the "post" of channels_ready to the start of the multifd_send_thread()
loop and added a missing "wait" at multifd_send_sync_main(). While it
does work, the placement of the wait goes against what the rest of the
code does.

The sequence at multifd_send_thread() is:

    qemu_sem_post(&multifd_send_state->channels_ready);
    qemu_sem_wait(&p->sem);
    <work>
    if (flags & MULTIFD_FLAG_SYNC) {
        qemu_sem_post(&p->sem_sync);
    }

Which means that the sending thread makes itself available
(channels_ready) and waits for more work (sem). So the sequence in the
migration thread should be to check if any channel is available
(channels_ready), give it some work and set it off (sem):

    qemu_sem_wait(&multifd_send_state->channels_ready);
    <enqueue work>
    qemu_sem_post(&p->sem);
    if (flags & MULTIFD_FLAG_SYNC) {
        qemu_sem_wait(&p->sem_sync);
    }

The reason there's no deadlock today is that the migration thread
enqueues the SYNC packet right before the wait on channels_ready and
we end up taking advantage of the out-of-order post to sem:

        ...
        qemu_sem_post(&p->sem);
    }
    for (i = 0; i < migrate_multifd_channels(); i++) {
        MultiFDSendParams *p = &multifd_send_state->params[i];

        qemu_sem_wait(&multifd_send_state->channels_ready);
        trace_multifd_send_sync_main_wait(p->id);
        qemu_sem_wait(&p->sem_sync);
	...

Move the channels_ready wait before the sem post to keep the sequence
consistent. Also fix the error path to post to channels_ready and
sem_sync in the correct order.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/multifd.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index a7c7a947e3..d626740f2f 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -618,6 +618,7 @@ int multifd_send_sync_main(QEMUFile *f)
 
         trace_multifd_send_sync_main_signal(p->id);
 
+        qemu_sem_wait(&multifd_send_state->channels_ready);
         qemu_mutex_lock(&p->mutex);
 
         if (p->quit) {
@@ -635,7 +636,6 @@ int multifd_send_sync_main(QEMUFile *f)
     for (i = 0; i < migrate_multifd_channels(); i++) {
         MultiFDSendParams *p = &multifd_send_state->params[i];
 
-        qemu_sem_wait(&multifd_send_state->channels_ready);
         trace_multifd_send_sync_main_wait(p->id);
         qemu_sem_wait(&p->sem_sync);
 
@@ -763,8 +763,8 @@ out:
      * who pay attention to me.
      */
     if (ret != 0) {
-        qemu_sem_post(&p->sem_sync);
         qemu_sem_post(&multifd_send_state->channels_ready);
+        qemu_sem_post(&p->sem_sync);
     }
 
     qemu_mutex_lock(&p->mutex);
-- 
2.35.3