[PATCH] mm: fix kmap_high deadlock V2

[zhangchun <zhang.chuna@h3c.com>]

I work with zhangzhansheng(from h3c) find that some situation
may casue deadlock when we use kmap_high and kmap_XXX or kumap_xxx
between  differt cores at the same time,  kmap_high->map_new_virtual->
flush_all_zero_pkmaps->flush_tlb_kernel_range->on_each_cpu. On this
condition, kmap_high hold the kmap_lock，wait the other cores respond
to ipi. But the others may disable irq and wait kmap_lock,
this is some deadlock condition. I think it's necessary to give
kmap_lock before call flush_tlb_kernel_range.
Like this:
spin_unlock(&kmap_lock);
flush_tlb_kernel_range(xxx);
spin_lock(&kmap_lock);

CPU 0:                cpu 1:
			kmap_xxx() {
xxx			irq_disable();
kmap_high();		spin_lock(&kmap_lock)
xxx			yyyyyyy
			spin_unlock(&kmap_lock)
			irq_enable();
			}
kmap_high detail:
kmap_high() {
	zzz
	spin_lock(&kmap_lock)
	map_new_virtual->
		flush_all_zero_pkmaps->
			flush_tlb_kernel_range->
				on_each_cpu
	/*
	if cpu 1 irq_disabled, the cpu 1
	cannot ack, then cpu 0 and cpu 1 may hangup.
	*/
	spin_unlock(&kmap_lock)
	zzz
}
Signed-off-by: zhangchun <zhang.chuna@h3c.com>

Reviewed-by: zhangzhengming <zhang.zhengming@h3c.com>
---
 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