[PATCH v2 10/17] migration/multifd: Convert multifd_send()::next_channel to atomic

[Maciej S. Szmigiero]

From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>

This is necessary for multifd_send() to be able to be called
from multiple threads.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
---
 migration/multifd.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index d5a8e5a9c9b5..b25789dde0b3 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -343,26 +343,38 @@ bool multifd_send(MultiFDSendData **send_data)
         return false;
     }
 
-    /* We wait here, until at least one channel is ready */
-    qemu_sem_wait(&multifd_send_state->channels_ready);
-
     /*
      * next_channel can remain from a previous migration that was
      * using more channels, so ensure it doesn't overflow if the
      * limit is lower now.
      */
-    next_channel %= migrate_multifd_channels();
-    for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) {
+    i = qatomic_load_acquire(&next_channel);
+    if (unlikely(i >= migrate_multifd_channels())) {
+        qatomic_cmpxchg(&next_channel, i, 0);
+    }
+
+    /* We wait here, until at least one channel is ready */
+    qemu_sem_wait(&multifd_send_state->channels_ready);
+
+    while (true) {
+        int i_next;
+
         if (multifd_send_should_exit()) {
             return false;
         }
+
+        i = qatomic_load_acquire(&next_channel);
+        i_next = (i + 1) % migrate_multifd_channels();
+        if (qatomic_cmpxchg(&next_channel, i, i_next) != i) {
+            continue;
+        }
+
         p = &multifd_send_state->params[i];
         /*
          * Lockless read to p->pending_job is safe, because only multifd
          * sender thread can clear it.
          */
         if (qatomic_read(&p->pending_job) == false) {
-            next_channel = (i + 1) % migrate_multifd_channels();
             break;
         }
     }