hugetlb: clear huge pte during flush function on mips platform

If multiple threads are accessing the same huge page at the same
time, hugetlb_cow will be called if one thread write the COW huge
page. And function huge_ptep_clear_flush is called to notify other
threads to clear the huge pte tlb entry. The other threads clear
the huge pte tlb entry and reload it from page table, the reload
huge pte entry may be old.

This patch fixes this issue on mips platform, and it clears huge
pte entry before notifying other threads to flush current huge
page entry, it is similar with other architectures.
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/hugetlb.h

@@ -46,7 +46,13 @@ static inline pte_t huge_ptep_get_and_clear(struct mm_struct *mm,
 static inline void huge_ptep_clear_flush(struct vm_area_struct *vma,
 					 unsigned long addr, pte_t *ptep)
 {
-	flush_tlb_page(vma, addr & huge_page_mask(hstate_vma(vma)));
+	/*
+	 * clear the huge pte entry firstly, so that the other smp threads will
+	 * not get old pte entry after finishing flush_tlb_page and before
+	 * setting new huge pte entry
+	 */
+	huge_ptep_get_and_clear(vma->vm_mm, addr, ptep);
+	flush_tlb_page(vma, addr);
 }
 
 #define __HAVE_ARCH_HUGE_PTE_NONE