Re: [PATCH v7 10/18] jobs: rename static functions called with job_mutex held

[Emanuele Giuseppe Esposito]

Am 23/06/2022 um 13:10 schrieb Vladimir Sementsov-Ogievskiy:
> On 6/23/22 12:08, Emanuele Giuseppe Esposito wrote:
>>
>>
>> Am 22/06/2022 um 20:38 schrieb Vladimir Sementsov-Ogievskiy:
>>> On 6/22/22 17:26, Emanuele Giuseppe Esposito wrote:
>>>>
>>>>
>>>> Am 21/06/2022 um 19:26 schrieb Vladimir Sementsov-Ogievskiy:
>>>>> On 6/16/22 16:18, Emanuele Giuseppe Esposito wrote:
>>>>>> With the*nop*  job_lock/unlock placed, rename the static
>>>>>> functions that are always under job_mutex, adding "_locked" suffix.
>>>>>>
>>>>>> List of functions that get this suffix:
>>>>>> job_txn_ref           job_txn_del_job
>>>>>> job_txn_apply           job_state_transition
>>>>>> job_should_pause       job_event_cancelled
>>>>>> job_event_completed       job_event_pending
>>>>>> job_event_ready           job_event_idle
>>>>>> job_do_yield           job_timer_not_pending
>>>>>> job_do_dismiss           job_conclude
>>>>>> job_update_rc           job_commit
>>>>>> job_abort           job_clean
>>>>>> job_finalize_single       job_cancel_async
>>>>>> job_completed_txn_abort       job_prepare
>>>>>> job_needs_finalize       job_do_finalize
>>>>>> job_transition_to_pending  job_completed_txn_success
>>>>>> job_completed           job_cancel_err
>>>>>> job_force_cancel_err
>>>>>>
>>>>>> Note that "locked" refers to the*nop*  job_lock/unlock, and not
>>>>>> real_job_lock/unlock.
>>>>>>
>>>>>> No functional change intended.
>>>>>>
>>>>>> Signed-off-by: Emanuele Giuseppe Esposito<eesposit@redhat.com>
>>>>>
>>>>>
>>>>> Hmm. Maybe it was already discussed.. But for me it seems, that it
>>>>> would
>>>>> be simpler to review previous patches, that fix job_ API users to use
>>>>> locking properly, if this renaming go earlier.
>>>>>
>>>>> Anyway, in this series, we can't update everything at once. So
>>>>> patch to
>>>>> patch, we make the code more and more correct. (yes I remember that
>>>>> lock() is a noop, but I should review thinking that it real,
>>>>> otherwise,
>>>>> how to review?)
>>>>>
>>>>> So, I'm saying about formal correctness of using lock() unlock()
>>>>> function in connection with introduced _locked prifixes and in
>>>>> connection with how it should finally work.
>>>>>
>>>>> You do:
>>>>>
>>>>> 05. introduce some _locked functions, that just duplicates, and
>>>>> job_pause_point_locked() is formally inconsistent, as I said.
>>>>>
>>>>> 06. Update a lot of places, to give them their final form (but not
>>>>> final, as some functions will be renamed to _locked, some not, hard to
>>>>> imagine)
>>>>>
>>>>> 07,08,09. Update some more, and even more places. very hard to track
>>>>> formal correctness of using locks
>>>>>
>>>>> 10-...: rename APIs.
>>>>>
>>>>>
>>>>> What do you think about the following:
>>>>>
>>>>> 1. Introduce noop lock, and some internal _locked() versions, and keep
>>>>> formal consistency inside job.c, considering all public interfaces as
>>>>> unlocked:
>>>>>
>>>>>    at this point:
>>>>>     - everything correct inside job.c
>>>>>     - no public interfaces with _locked prefix
>>>>>     - all public interfaces take mutex internally
>>>>>     - no external user take mutex by hand
>>>>>
>>>>> We can rename all internal static functions at this step too.
>>>>>
>>>>> 2. Introduce some public _locked APIs, that we'll use in next patches
>>>>>
>>>>> 3. Now start fixing external users in several patches:
>>>>>       - protect by mutex direct use of job fields
>>>>>     - make wider locks and move to _locked APIs inside them where
>>>>> needed
>>>>>
>>>>>
>>>>> In this scenario, every updated unit becomes formally correct after
>>>>> update, and after all steps everything is formally correct, and we can
>>>>> move to turning-on the mutex.
>>>>>
>>>>
>>>> I don't understand your logic also here, sorry :(
>>>>
>>>> I assume you want to keep patch 1-4, then the problem is assing
>>>> job_lock
>>>> and renaming functions in _locked.
>>>> So I would say the problem is in patch 5-6-10-11-12-13. All the others
>>>> should be self contained.
>>>>
>>>> I understand patch 5 is a little hard to follow.
>>>>
>>>> Now, I am not sure what you propose here but it seems that the end goal
>>>> is to just have the same result, but with additional intermediate steps
>>>> that are just "do this just because in the next patch will be useful".
>>>> I think the problem is that we are going to miss the "why we need the
>>>> lock" logic in the patches if we do so.
>>>>
>>>> The logic I tried to convey in this order is the following:
>>>> - job.h: add _locked duplicates for job API functions called with and
>>>> without job_mutex
>>>>      Just create duplicates of functions
>>>>
>>>> - jobs: protect jobs with job_lock/unlock
>>>>      QMP and monitor functions call APIs that assume lock is taken,
>>>>      drivers must take explicitly the lock
>>>>
>>>> - jobs: rename static functions called with job_mutex held
>>>> - job.h: rename job API functions called with job_mutex held
>>>> - block_job: rename block_job functions called with job_mutex held
>>>>      *given* that some functions are always under lock, transform
>>>>      them in _locked. Requires the job_lock/unlock patch
>>>>
>>>> - job.h: define unlocked functions
>>>>      Comments on the public functions that are not _locked
>>>>
>>>>
>>>> @Kevin, since you also had some feedbacks on the patch ordering, do you
>>>> agree with this ordering or you have some other ideas?
>>>>
>>>> Following your suggestion, we could move patches 10-11-12-13 before
>>>> patch 6 "jobs: protect jobs with job_lock/unlock".
>>>>
>>>> (Apologies for changing my mind, but being the second complain I am
>>>> starting to reconsider reordering the patches).
>>>>
>>>
>>> In two words, what I mean: let's keep the following invariant from patch
>>> to patch:
>>>
>>> 1. Function that has _locked() prefix is always called with lock held
>>> 2. Function that has _locked() prefix never calls functions that take
>>> lock by themselves so that would dead-lock
>>> 3. Function that is documented as "called with lock not held" is never
>>> called with lock held
>>>
>>> That what I mean by "formal correctness": yes, we know that lock is
>>> noop, but still let's keep code logic to correspond function naming and
>>> comments that we add.
>>>
>>
>> Ok I get what you mean, but then we have useless changes for public
>> functions that eventually will only be _locked() like job_next_locked:
>>
>> The function is always called in a loop, so it is pointless to take the
>> lock inside. Therefore the patch would be "incorrect" on its own anyways.
>>
>> Then, we would have a patch where we add the lock guard inside, and
>> another one where we remove it and rename to _locked and take the lock
>> outside. Seems unnecessary to me.
> 
> For me it looks a bit simpler than you describe. And anyway keeping the
> correctness from patch to patch worth the complexity. I'll give an
> argument.
> 
> First what is the best practices? Best practices is when every patch is
> good and absolutely correct. So that you can apply any number of patches
> from the beginning of the series (01-NN), commit them to master and this
> will break neither compilation, nor tests, nor readability, nothing.
> This makes the review process iterable: if I'm OK with patches 01-03, I
> give them r-b and don't think about them. I don't have to keep in mind
> any tricky things. And I can review 04 several days later not rereading
> 01-03 (or at least I can consider applied 01-03 as a good correct base
> state). This way I'm sure, that if I reviewed all patches one-by-one,
> each one is correct, then the whole thing is correct.
> 
> A lot harder to review when we have only collective correctness: the
> whole series being applied make a correct thing, but we can't say it
> about intermediate states. In your series we can't be absolutely correct
> with each patch, as we have to switch from aio-context lock to mutex in
> one patch, that's why mutex is added as noop. That's a reasonable and
> (seems) unsolvable drawback. That's a thing I have to keep in mind
> during the whole review. But I'd prefer not add more such things, like
> comments and _locked suffixes that don't correspond to the code.
> 
> With the invariant that I propose, the following logic works:
> 
> If
>    1. we keep the invariant from patch to patch
>    AND
>    2. at the end we have updated all users of the internal and external
> APIs, not missed some file or function
> Then everything is correct at the end.
> 
> Without the invariant I can't prove that everything is correct at the
> end, as it is hard to follow the degree of correctness from patch to
> patch. In your way the only invariant that we have from patch to patch,
> is that mutex is noop, so all changes do nothing, and therefore they are
> correct. This way I can give an r-b to all such patches not thinking
> about details, they are noop. But when I finally have to review the
> patch that turns on the mutex, I'll have to recheck all internal and
> external API users, which is equivalent to review all the changes merged
> into one patch.
> 
> 
> 
> Consider the case with job_next. The most correct way to update it IMHO:
> 
> 1. Add lock inside job_next() and add job_next_locked() - in one patch
> with other similar changes of job.c and job.h.
> 
> At this moment we have job_next() calls in a loop, which is not good (we
> want larger critical section), but that doesn't break the invariant I
> proposed above.

The only thing I am pointing here is that this breaks "readability",
meaning if someone bisects here it will find a very weird situation
(aside from the fact that there is a noop lock).

But I guess this is fine, as long as I write it in the commit message.

And since these patch are waiting here for more than 3 months now, I
would say if the others (Kevin?) agree I will change the order with what
you proposed here.

Emanuele

> 
> 2. Update the loop: add larger critical section and switch to
> job_next_locked().
> 
> What is good here: we don't  need to unite updates of external API users
> into one patch, we can update file-by-file or subsystem-by-sybsystem.
> 
> 3. Delete unused job_next() API
> 
>