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Subject: Re: [virtio-dev] Re: [PATCH v9 3/8] migration: use bitmap_mutex in migration_bitmap_clear_dirty
On 11/27/2018 02:02 PM, Wei Wang wrote:
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) { rs->migration_dirty_pages--; } + 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.
And before this feature, I think yes, that bitmap_mutex is not needed. It was left there due to some historical reasons. I remember Dave previous said he was about to remove it. But the new feature will need it again. Best, Wei
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