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

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Date:   Mon, 20 Jun 2022 09:56:06 +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 2022/6/18 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.

For spin_lock, they are lock(A) lock(B) unlock(B) unlock(A) e.g. in copy_huge_pud,
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!

> 
> .
>