[PATCH] mm: Give kmap_lock before call flush_tlb_kernel_rang，avoid kmap_high deadlock.

[zhangchun]

Use kmap_high and kmap_XXX or kumap_xxx among differt cores at the same
time may cause deadlock. The issue is like this：

 CPU 0:                                                 CPU 1:
 kmap_high(){                                           kmap_xxx() {
               ...                                        irq_disable();
        spin_lock(&kmap_lock)
               ...
        map_new_virtual                                     ...
           flush_all_zero_pkmaps
              flush_tlb_kernel_range         /* CPU0 holds the kmap_lock */
                      smp_call_function_many         spin_lock(&kmap_lock)
                      ...                                   ....
        spin_unlock(&kmap_lock)
               ...

CPU 0 holds the kmap_lock, waiting for CPU 1 respond to IPI. But CPU 1
has disabled irqs, waiting for kmap_lock, cannot answer the IPI. Fix
this by releasing  kmap_lock before call flush_tlb_kernel_range,
avoid kmap_lock deadlock.

Fixes: 3297e760776a ("highmem: atomic highmem kmap page pinning")
Signed-off-by: zhangchun <zhang.chuna@xxxxxxx>
Co-developed-by: zhangzhansheng <zhang.zhansheng@xxxxxxx>
Signed-off-by: zhangzhansheng <zhang.zhansheng@xxxxxxx>
Reviewed-by: zhangzhengming <zhang.zhengming@xxxxxxx>
---
 mm/highmem.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/highmem.c b/mm/highmem.c
index bd48ba4..841b370 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -220,8 +220,11 @@ static void flush_all_zero_pkmaps(void)
 		set_page_address(page, NULL);
 		need_flush = 1;
 	}
-	if (need_flush)
+	if (need_flush) {
+		spin_unlock(&kmap_lock);
 		flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));
+		spin_lock(&kmap_lock);
+	}
 }
 
 void __kmap_flush_unused(void)
-- 
1.8.3.1