Although accesses to ram_list.dirty_memory[] use atomics so multiple
threads can safely dirty the bitmap, the data structure is not fully
thread-safe yet.
This patch handles the RAM hotplug case where ram_list.dirty_memory[] is
grown. ram_list.dirty_memory[] is change from a regular bitmap to an
RCU array of pointers to fixed-size bitmap blocks. Threads can continue
accessing bitmap blocks while the array is being extended. See the
comments in the code for an in-depth explanation of struct
DirtyMemoryBlocks.
I have tested that live migration with virtio-blk dataplane works.
Backports commit 5b82b703b69acc67b78b98a5efc897a3912719eb from qemu
The new bitmap_test_and_clear_atomic() function clears a range and
returns whether or not the bits were set.
Backports commit 36546e5b803f6e363906607307f27c489441fd15 from qemu
Use atomic_or() for atomic bitmaps where several threads may set bits at
the same time. This avoids the race condition between threads loading
an element, bitwise ORing, and then storing the element.
When setting all bits in a word we can avoid atomic ops and instead just
use an smp_mb() at the end.
Most bitmap users don't need atomicity so introduce new functions.
Backports commit 9f02cfc84b85929947b32fe1674fbc6a429f332a from qemu
