[RFC PATCH 3/3] Restart pathwalk on rename seqcount change

[Tingmao Wang]

On 6/4/25 19:56, Tingmao Wang wrote:
> On 6/4/25 03:21, Al Viro wrote:
>> On Wed, Jun 04, 2025 at 02:12:11AM +0100, Tingmao Wang wrote:
>>> On 6/4/25 01:55, Al Viro wrote:
>>>> On Wed, Jun 04, 2025 at 01:45:45AM +0100, Tingmao Wang wrote:
>>>>> +		rename_seqcount = read_seqbegin(&rename_lock);
>>>>> +		if (rename_seqcount % 2 == 1) {
>>>>
>>>> Please, describe the condition when that can happen, preferably
>>>> along with a reproducer.
>>>
>>> My understanding is that when a rename is in progress the seqcount is odd,
>>> is that correct?
>>>
>>> If that's the case, then the fs_race_test in patch 2 should act as a
>>> reproducer, since it's constantly moving the directory.
>>>
>>> I can add a comment to explain this, thanks for pointing out.
>>
>> Please, read through the header declaring those primitives and read the
>> documentation it refers to - it's useful for background.
> 
> Ok, so I didn't realize read_seqbegin actually waits for the seqcount to
> turn even.  I did read the header earlier when following dget_parent but
> probably misremembered and mixed raw_seqcount_begin with read_seqbegin.

Right, after more careful looking I think what I actually want here is
raw_read_seqcount.  My apologies.

> 
>>
>> What's more, look at the area covered by rename_lock - I seriously suspect
>> that you are greatly overestimating it.
> 
> Admittedly "when a rename is in progress" is a vague statement.  Looking
> at what takes rename_lock in the code, it's only when we actually do
> d_move where we take this lock (plus some other places), and the critical
> section isn't very large, and does not contain any waits etc.
> 
> If we keep read_seqbegin, then that gives landlock more opportunity to do
> a reference-less parent walk, but at the expense that a d_move anywhere,
> even if it doesn't affect anything we're currently looking at, will
> temporarily block this landlocked application (even if not for very long),
> and multiple concurrent renames might cause us to wait for longer (but
> probably won't starve us since we just need one "cycle" where rename
> seqcount is even).
> 
> Since we can still safely do a parent walk, just needing to take dentry
> references on our way, we could simply fallback to that in this situation.
> i.e.  we can use raw_seqcount_begin and keep the seqcount & 1 check.

This will be raw_read_seqcount(&rename_lock.seqcount)

> 
> Now, there is the argument that if d_move is very quick, then it might be
> worth waiting for it to finish, and we will fallback to the original
> parent walk if the seqcount changes again.  I'm not sure which is best,
> but I'm inclining towards changing this to raw_seqcount_begin, as this is
> purely an optimization, and we do not _need_ to avoid concurrent renames.