linux-kernel - Re: [PATCH] filemap: obey mapping->invalidate

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Date:   Mon, 20 Jun 2022 14:35:30 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] filemap: obey mapping->invalidate_lock lock/unlock order

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?

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!
>>
>>>
>>> .
>>>
>>
> 
> .
>