Re: [PATCH] filemap: obey mapping->invalidate_lock lock/unlock order

From: Miaohe Lin
Date: Sun Jun 19 2022 - 21:56:15 EST


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!

>
> .
>