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Date:   Mon, 20 Jun 2022 17:47:28 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     Miaohe Lin <linmiaohe@huawei.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
Subject: Re: [PATCH] filemap: obey mapping->invalidate_lock lock/unlock order
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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.
 
> 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!
> >>
> >>>
> >>> .
> >>>
> >>
> > 
> > .
> > 
> 
>