[PATCH v2] 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：


>What is kmap_XXX?

kmap/kunmap.

>>  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")

>Wow, that's 15 years old.  Has the deadlock been observed?

Yeah, the device crashed due to this reason. 
 
>> --- 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) {
>> +		unlock_kmap();
>>  		flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));
>> +		lock_kmap();
>> +	}
>>  }

>Why is dropping the lock like this safe?  What data is it protecting and why is it OK to 
>leave that data unprotected here?

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.
-- 
1.8.3.1