xen/mmu: WARN_ON when racing to swap middle leaf.

The initial bootup code uses set_phys_to_machine quite a lot, and after
bootup it would be used by the balloon driver. The balloon driver does have
mutex lock so this should not be necessary - but just in case, add
a WARN_ON if we do hit this scenario. If we do fail this, it is OK
to continue as there is a backup mechanism (VM_IO) that can bypass
the P2M and still set the _PAGE_IOMAP flags.

[v2: Change from WARN to BUG_ON]
[v3: Rebased on top of xen->p2m code split]
[v4: Change from BUG_ON to WARN]
Reviewed-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 809fe3536301..4631cf99e714 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -605,7 +605,8 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 
                 /* Swap over from MISSING to IDENTITY if needed. */
                 if (p2m_top[topidx][mididx] == p2m_missing) {
-                        p2m_top[topidx][mididx] = p2m_identity;
+                        WARN_ON(cmpxchg(&p2m_top[topidx][mididx], p2m_missing,
+                                p2m_identity) != p2m_missing);
                         return true;
                 }
         }