On 2022/6/20 17:47, Muchun Song wrote: > 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. Yes, it seems the gain is not worth complicating the code. So I will drop the patch. Thanks. > >> 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! >>>> >>>>> >>>>> . >>>>> >>>> >>> >>> . >>> >> >> > . >
