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SubjectRe: [PATCH v1] proc/vmcore: fix false positive lockdep warning
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On 19.01.22 16:15, David Hildenbrand wrote:
> On 19.01.22 16:08, Boqun Feng wrote:
>> Hi,
>>
>> On Wed, Jan 19, 2022 at 12:37:02PM +0100, David Hildenbrand wrote:
>>> Lockdep complains that we do during mmap of the vmcore:
>>> down_write(mmap_lock);
>>> down_read(vmcore_cb_rwsem);
>>> And during read of the vmcore:
>>> down_read(vmcore_cb_rwsem);
>>> down_read(mmap_lock);
>>>
>>> We cannot possibly deadlock when only taking vmcore_cb_rwsem in read
>>> mode, however, it's hard to teach that to lockdep.
>>>
>>
>> Lockdep warned about the above sequences because rw_semaphore is a fair
>> read-write lock, and the following can cause a deadlock:
>>
>> TASK 1 TASK 2 TASK 3
>> ====== ====== ======
>> down_write(mmap_lock);
>> down_read(vmcore_cb_rwsem)
>> down_write(vmcore_cb_rwsem); // blocked
>> down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness
>> down_read(mmap_lock); // blocked
>>
>> IOW, a reader can block another read if there is a writer queued by the
>> second reader and the lock is fair.
>>
>> So there is a deadlock possiblity.
>
> Task 3 will never take the mmap_lock before doing a
> down_write(vmcore_cb_rwsem).
>
> How would this happen?

Ah, I get it, nevermind. I'll adjust the patch description.

Thanks!

--
Thanks,

David / dhildenb

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