[PATCH 10/10] xen/mmu: Warn against races.

[Konrad Rzeszutek Wilk]

The initial bootup code uses set_phys_to_machine quite a lot, and after
bootup it would be 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@xxxxxxxxxx>
---
 arch/x86/xen/mmu.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index b2b8733..86d3bd3 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -549,13 +549,15 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 	idx = p2m_index(pfn);
 
 	if (mfn == INVALID_P2M_ENTRY) {
-		/* If it is INVALID, swap over.. */
+		/* Ballon lock should inhibit racing, but just in case.*/
 		if (p2m_top[topidx] == p2m_mid_identity) {
-			p2m_top[topidx] = p2m_mid_missing;
+			WARN_ON(cmpxchg(&p2m_top[topidx], p2m_mid_identity,
+				p2m_mid_missing) != p2m_mid_identity);
 			return 1;
 		}
 		if (p2m_top[topidx][mididx] == p2m_identity) {
-			p2m_top[topidx][mididx] = p2m_missing;
+			WARN_ON(cmpxchg(&p2m_top[topidx][mididx], p2m_identity,
+				p2m_missing) != p2m_identity);
 			return 1;
 		}
 	}
-- 
1.7.1

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