[PATCH v4] mm: Give kmap_lock before call flush_tlb_kernel_rang，avoid kmap_high deadlock.

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

 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.

        if (need_flush) {
            unlock_kmap();
            flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));
            lock_kmap();
        }

Dropping the lock like this is safe. kmap_lock is used to protect pkmap_count, pkmap_page_table and last_pkmap_nr(static variable). 
When call flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP)), flush_tlb_kernel_range will neither modify nor read these variables. 
Leave that data unprotected here is safe.

map_new_virtual aims to find an usable entry pkmap_count[last_pkmap_nr].
When read and modify the pkmap_count[last_pkmap_nr], the kmap_lock is not dropped. "if (!pkmap_count[last_pkmap_nr])" determine pkmap_count[last_pkmap_nr] is usable or not. 
If unusable, try agin.

Furthermore, the value of static variable last_pkmap_nr is stored in a local variable last_pkmap_nr, when kmap_lock is acquired, this is thread-safe.

In an extreme case, if Thread A and Thread B access the same last_pkmap_nr, Thread A calls function flush_tlb_kernel_range and release the kmap_lock,
and Thread B then acquires the kmap_lock and modifies the variable pkmap_count[last_pkmap_nr]. After Thread A completes the execution of function the
variable pkmap_count[last_pkmap_nr]. After Thread A completes the execution of function flush_tlb_kernel_range, it will check the variable 
pkmap_count[last_pkmap_nr].

static inline unsigned long map_new_virtual(struct page *page) {
        unsigned long vaddr;
        int count;
        unsigned int last_pkmap_nr; // local variable to store static variable last_pkmap_nr
        unsigned int color = get_pkmap_color(page);

start:
      ...
                        flush_all_zero_pkmaps();// release kmap_lock, then acquire it
                        count = get_pkmap_entries_count(color);
                }
                ...
                if (!pkmap_count[last_pkmap_nr]) // pkmap_count[last_pkmap_nr] is used or not
                        break;  /* Found a usable entry */
                if (--count)
                        continue;

               ...
        vaddr = PKMAP_ADDR(last_pkmap_nr);
        set_pte_at(&init_mm, vaddr,
                   &(pkmap_page_table[last_pkmap_nr]), mk_pte(page, kmap_prot));

        pkmap_count[last_pkmap_nr] = 1;
        ...
        return vaddr;
}

Fixes: 3297e760776a ("highmem: atomic highmem kmap page pinning")
Signed-off-by: zhangchun <zhang.chuna@h3c.com>
Co-developed-by: zhangzhansheng <zhang.zhansheng@h3c.com>
Signed-off-by: zhangzhansheng <zhang.zhansheng@h3c.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: zhangzhengming <zhang.zhengming@h3c.com>
---
 mm/highmem.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/highmem.c b/mm/highmem.c index ef3189b..07f2c67 100644
--- a/mm/highmem.c
+++ b/mm/highmem.c
@@ -231,8 +231,18 @@ static void flush_all_zero_pkmaps(void)
 		set_page_address(page, NULL);
 		need_flush = 1;
 	}
-	if (need_flush)
+	if (need_flush) {
+		/*
+		 * In multi-core system one CPU holds the kmap_lock, waiting
+		 * for other CPUs respond to IPI. But other CPUS has disabled
+		 * irqs, waiting for kmap_lock, cannot answer the IPI. Release
+		 * kmap_lock before call flush_tlb_kernel_range, avoid kmap_lock
+		 * deadlock.
+		 */
+		unlock_kmap();
 		flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));
+		lock_kmap();
+	}
 }
 
 void __kmap_flush_unused(void)
--
1.8.3.1


-- 
1.8.3.1