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