lkml.org 
[lkml]   [2022]   [Jun]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] filemap: obey mapping->invalidate_lock lock/unlock order
From
Date
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!
>>
>>>
>>> .
>>>
>>
>
> .
>

\
 
 \ /
  Last update: 2022-06-20 08:36    [W:0.066 / U:0.080 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site