>> 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.
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1.8.3.1
