[PATCH] migration: fix multifd_send_pages() next channel

[Laurent Vivier <lvivier@redhat.com>]

multifd_send_pages() loops around the available channels,
the next channel to use between two calls to multifd_send_pages() is stored
inside a local static variable, next_channel.

It works well, except if the number of channels decreases between two calls
to multifd_send_pages(). In this case, the loop can try to access the
data of a channel that doesn't exist anymore.

The problem can be triggered if we start a migration with a given number of
channels and then we cancel the migration to restart it with a lower number.
This ends generally with an error like:
qemu-system-ppc64: .../util/qemu-thread-posix.c:77: qemu_mutex_lock_impl: Assertion `mutex->initialized' failed.

This patch fixes the error by capping next_channel with the current number
of channels before using it.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 migration/multifd.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/migration/multifd.c b/migration/multifd.c
index 5a3e4d0d46d1..d0441202aae9 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -415,6 +415,12 @@ static int multifd_send_pages(QEMUFile *f)
     }
 
     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()) {
         p = &multifd_send_state->params[i];
 
-- 
2.26.2