[PATCH v2 bpf-next] arm64: mm: Complete the PTE store in ptep_try_set()

[Tejun Heo]

ptep_try_set() installs a kernel PTE with try_cmpxchg() but, unlike
__set_pte(), skips the barriers that arm64 requires after writing a valid
kernel PTE. Without them a subsequent access can fault instead of seeing
the new mapping.

Issue them with emit_pte_barriers() rather than __set_pte_complete().
ptep_try_set() must finish the store before it returns, but
__set_pte_complete() would defer the barriers when the calling context is in
lazy MMU mode.

v2: Emit the barriers directly instead of __set_pte_complete(). (Catalin)

Fixes: 258df8fce42f ("mm: Add ptep_try_set() for lockless empty-slot installs")
Suggested-by: Catalin Marinas <catalin.marinas at arm.com>
Link: https://lore.kernel.org/all/aiRFcz78QTZdIHHB@arm.com/
Signed-off-by: Tejun Heo <tj at kernel.org>
---
 arch/arm64/include/asm/pgtable.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 3ce0f2a6cab6..3e579c26b383 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1838,7 +1838,16 @@ static inline bool ptep_try_set(pte_t *ptep, pte_t new_pte)
 {
 	pteval_t old = 0;
 
-	return try_cmpxchg(&pte_val(*ptep), &old, pte_val(new_pte));
+	if (!try_cmpxchg(&pte_val(*ptep), &old, pte_val(new_pte)))
+		return false;
+
+	/*
+	 * The store must be complete by the time this returns, but the caller
+	 * may be in lazy MMU mode, where __set_pte_complete() would defer the
+	 * barriers. Issue them directly.
+	 */
+	emit_pte_barriers();
+	return true;
 }
 #define ptep_try_set ptep_try_set
 
-- 
2.51.1