Re: [PATCH v5 0/2] tcp/dcpp: Un-pin tw_timer

[Valentin Schneider]

Apologies for the delayed reply, I was away for most of last week;

On 16/04/24 17:01, Eric Dumazet wrote:
> On Mon, Apr 15, 2024 at 4:33 PM Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
>>
>> On 15/04/24 14:35, Eric Dumazet wrote:
>> > On Mon, Apr 15, 2024 at 1:34 PM Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
>> >> v4 -> v5
>> >> ++++++++
>> >>
>> >> o Rebased against latest Linus' tree
>> >> o Converted tw_timer into a delayed work following Jakub's bug report on v4
>> >>   http://lore.kernel.org/r/20240411100536.224fa1e7@xxxxxxxxxx
>> >
>> > What was the issue again ?
>> >
>> > Please explain precisely why it was fundamentally tied to the use of
>> > timers (and this was not possible to fix the issue without
>> > adding work queues and more dependencies to TCP stack)
>>
>> In v4 I added the use of the ehash lock to serialize arming the timewait
>> timer vs destroying it (inet_twsk_schedule() vs inet_twsk_deschedule_put()).
>>
>> Unfortunately, holding a lock both in a timer callback and in the context
>> in which it is destroyed is invalid. AIUI the issue is as follows:
>>
>>   CPUx                        CPUy
>>   spin_lock(foo);
>>                               <timer fires>
>>                               call_timer_fn()
>>                                 spin_lock(foo) // blocks
>>   timer_shutdown_sync()
>>     __timer_delete_sync()
>>       __try_to_del_timer_sync() // looped as long as timer is running
>>                        <deadlock>
>>
>> In our case, we had in v4:
>>
>>   inet_twsk_deschedule_put()
>>     spin_lock(ehash_lock);
>>                                           tw_timer_handler()
>>                                             inet_twsk_kill()
>>                                               spin_lock(ehash_lock);
>>                                               __inet_twsk_kill();
>>     timer_shutdown_sync(&tw->tw_timer);
>>
>> The fix here is to move the timer deletion to a non-timer
>> context. Workqueues fit the bill, and as the tw_timer_handler() would just queue
>> a work item, I converted it to a delayed_work.
>
> I do not like this delayed work approach.
>
> Adding more dependencies to the TCP stack is not very nice from a
> maintainer point of view.
>
> Why couldn't you call timer_shutdown_sync() before grabbing the lock ?

We need the timer_shutdown_sync() and mod_timer() of tw->tw_timer to be
serialized in some way. If they aren't, we have the following race:

                             tcp_time_wait()
                               inet_twsk_hashdance()
  inet_twsk_deschedule_put()
    // Returns 0 because not pending, but prevents future arming
    timer_shutdown_sync()
                               inet_twsk_schedule()
                                 // Returns 0 as if timer had been succesfully armed
                                 mod_timer()

This means inet_twsk_deschedule_put() doesn't end up calling
inet_twsk_kill() (because the timer wasn't pending when it got shutdown),
but inet_twsk_schedule() doesn't arm it either despite the hashdance()
having updated the refcounts.

If we leave the deschedule as a del_timer_sync(), the timer ends up armed
in inet_twsk_schedule(), but that means waiting for the timer to fire to
clean up the resources despite having called inet_twsk_deschedule_put().