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KVM: Fix __set_bit() race in mark_page_dirty() during dirty logging

It is possible that the __set_bit() in mark_page_dirty() is called
simultaneously on the same region of memory, which may result in only
one bit being set, because some callers do not take mmu_lock before
mark_page_dirty().

This problem is hard to produce because when we reach mark_page_dirty()
beginning from, e.g., tdp_page_fault(), mmu_lock is being held during
__direct_map():  making kvm-unit-tests' dirty log api test write to two
pages concurrently was not useful for this reason.

So we have confirmed that there can actually be race condition by
checking if some callers really reach there without holding mmu_lock
using spin_is_locked():  probably they were from kvm_write_guest_page().

To fix this race, this patch changes the bit operation to the atomic
version:  note that nr_dirty_pages also suffers from the race but we do
not need exactly correct numbers for now.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
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Takuya Yoshikawa Avi Kivity
Takuya Yoshikawa authored and Avi Kivity committed Jan 4, 2012
1 parent f8275f9 commit 50e92b3c971129c96a5fffb51dd42691e2ee4004
Showing with 1 addition and 1 deletion.
  1. +1 −1 virt/kvm/kvm_main.c
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@@ -1543,7 +1543,7 @@ void mark_page_dirty_in_slot(struct kvm *kvm, struct kvm_memory_slot *memslot,
if (memslot && memslot->dirty_bitmap) {
unsigned long rel_gfn = gfn - memslot->base_gfn;
- if (!__test_and_set_bit_le(rel_gfn, memslot->dirty_bitmap))
+ if (!test_and_set_bit_le(rel_gfn, memslot->dirty_bitmap))
memslot->nr_dirty_pages++;
}
}

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