[PATCH 4/8] xen/mmu: Warn against races.

[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 warning if we do hit this scenario.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 arch/x86/xen/mmu.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 667901f..36bdd2d 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -553,11 +553,13 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 
 		/* Swap over from MISSING to IDENTITY if needed. */
 		if (p2m_top[topidx] == p2m_mid_missing) {
-			p2m_top[topidx] = p2m_mid_identity;
+			WARN_ON(cmpxchg(&p2m_top[topidx], p2m_mid_missing,
+				p2m_mid_identity) != p2m_mid_missing);
 			return 1;
 		}
 		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 1;
 		}
 	}
-- 
1.7.1