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Subject: Re: [PATCH v9 3/8] migration: use bitmap_mutex in migration_bitmap_clear_dirty

On 11/27/2018 01:40 PM, Peter Xu wrote:
On Thu, Nov 15, 2018 at 06:07:59PM +0800, Wei Wang wrote:
The bitmap mutex is used to synchronize threads to update the dirty
bitmap and the migration_dirty_pages counter. For example, the free
page optimization clears bits of free pages from the bitmap in an
iothread context. This patch makes migration_bitmap_clear_dirty update
the bitmap and counter under the mutex.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Michael S. Tsirkin <mst@redhat.com>
CC: Peter Xu <peterx@redhat.com>
  migration/ram.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/migration/ram.c b/migration/ram.c
index 7e7deec..ef69dbe 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1556,11 +1556,14 @@ static inline bool migration_bitmap_clear_dirty(RAMState *rs,
      bool ret;
+ qemu_mutex_lock(&rs->bitmap_mutex);
      ret = test_and_clear_bit(page, rb->bmap);
if (ret) {
+    qemu_mutex_unlock(&rs->bitmap_mutex);
      return ret;
It seems fine to me, but have you thought about
test_and_clear_bit_atomic()?  Note that we just had
test_and_set_bit_atomic() a few months ago.

Thanks for sharing. I think we might also need to
mutex migration_dirty_pages.

And not related to this patch: I'm unclear on why we have had
bitmap_mutex before, since it seems unnecessary.

OK. This is because with the optimization we have a thread
which clears bits (of free pages) from the bitmap and updates
migration_dirty_pages. So we need to synchronization between
the migration thread and the optimization thread.


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