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Subject: Re: [PATCH] filemap: obey mapping->invalidate_lock lock/unlock order
To:     Muchun Song <songmuchun@bytedance.com>
CC:     Matthew Wilcox <willy@infradead.org>, <akpm@linux-foundation.org>,
        <linux-fsdevel@vger.kernel.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
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From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <385dcf89-da4c-d3ac-333a-7ab40ecbdf9f@huawei.com>
Date:   Mon, 20 Jun 2022 20:14:14 +0800
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On 2022/6/20 17:47, Muchun Song wrote:
> On Mon, Jun 20, 2022 at 02:35:30PM +0800, Miaohe Lin wrote:
>> On 2022/6/20 12:47, Matthew Wilcox wrote:
>>> On Mon, Jun 20, 2022 at 09:56:06AM +0800, Miaohe Lin wrote:
>>>> On 2022/6/18 18:34, Matthew Wilcox wrote:
>>>>> On Sat, Jun 18, 2022 at 04:38:20PM +0800, Miaohe Lin wrote:
>>>>>> The invalidate_locks of two mappings should be unlocked in reverse order
>>>>>> relative to the locking order in filemap_invalidate_lock_two(). Modifying
>>>>>
>>>>> Why?  It's perfectly valid to lock(A) lock(B) unlock(A) unlock(B).
>>>>> If it weren't we'd have lockdep check it and complain.
>>
>> It seems I misunderstand your word. I thought you said it must be at lock(A) lock(B) unlock(A) unlock(B)
>> order... Sorry.
>>
>>>>
>>>> For spin_lock, they are lock(A) lock(B) unlock(B) unlock(A) e.g. in copy_huge_pud,
>>>
>>> I think you need to spend some time thinking about the semantics of
>>> locks and try to figure out why it would make any difference at all
>>> which order locks (of any type) are _unlocked_ in,
>>
>> IIUC, the lock orders are important to prevent possible deadlock. But unlock orders should be relaxed
>> because they won't result in problem indeed. And what I advocate here is that making it at lock(A) lock(B)
>> unlock(B) unlock(A) order should be a better program practice. Or unlock order shouldn't be obligatory
>> at practice?
>>
> 
> lock(A) lock(B) unlock(A) unlock(B) is fine. So it is better not to complicate the code.

Yes, it seems the gain is not worth complicating the code. So I will drop the patch.

Thanks.

>  
>> Thanks.
>>
>>>
>>>> copy_huge_pmd, move_huge_pmd and so on:
>>>> 	dst_ptl = pmd_lock(dst_mm, dst_pmd);
>>>> 	src_ptl = pmd_lockptr(src_mm, src_pmd);
>>>> 	spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
>>>> 	...
>>>> 	spin_unlock(src_ptl);
>>>> 	spin_unlock(dst_ptl);
>>>>
>>>> For rw_semaphore, they are also lock(A) lock(B) unlock(B) unlock(A) e.g. in dup_mmap():
>>>> 	mmap_write_lock_killable(oldmm)
>>>> 	mmap_write_lock_nested(mm, SINGLE_DEPTH_NESTING);
>>>> 	...
>>>> 	mmap_write_unlock(mm);
>>>> 	mmap_write_unlock(oldmm);
>>>>
>>>> and ntfs_extend_mft():
>>>> 	down_write(&ni->file.run_lock);
>>>> 	down_write_nested(&sbi->used.bitmap.rw_lock, BITMAP_MUTEX_CLUSTERS);
>>>> 	...
>>>> 	up_write(&sbi->used.bitmap.rw_lock);
>>>> 	up_write(&ni->file.run_lock);
>>>>
>>>> But I see some lock(A) lock(B) unlock(A) unlock(B) examples in some fs codes. Could you
>>>> please tell me the right lock/unlock order? I'm somewhat confused now...
>>>>
>>>> BTW: If lock(A) lock(B) unlock(A) unlock(B) is requested, filemap_invalidate_lock_two might
>>>> still need to be changed to respect that order?
>>>>
>>>> Thanks!
>>>>
>>>>>
>>>>> .
>>>>>
>>>>
>>>
>>> .
>>>
>>
>>
> .
>