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migration: fix multifd_send_pages() next channel

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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>
Message-Id: <20200617113154.593233-1-lvivier@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
This commit is contained in:
Laurent Vivier 2020-06-17 13:31:54 +02:00 committed by Dr. David Alan Gilbert
parent 6bcd361a52
commit 7e89a1401a
1 changed files with 6 additions and 0 deletions
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6
migration/multifd.c
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@ -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];