[PATCH 04/11] xen/mmu: BUG_ON when racing to swap middle leaf.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

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 BUG_ON if we do hit this scenario.

[v2: Change from WARN to BUG_ON]
[v3: Rebased on top of xen->p2m code split]
Reviewed-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 arch/x86/xen/p2m.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 19b0a65..fbbd2ab 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -478,7 +478,8 @@ 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;
+			BUG_ON(cmpxchg(&p2m_top[topidx][mididx], p2m_missing,
+				p2m_identity) != p2m_missing);
 			return true;
 		}
 	}
-- 
1.7.1