Re: rcu: WARNING in rcu_seq_end

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, Mar 7, 2017 at 3:27 PM, Boqun Feng <boqun.feng@gmail.com> wrote:
> On Tue, Mar 07, 2017 at 08:05:19AM +0100, Dmitry Vyukov wrote:
> [...]
>> >>
>> >> What is that mutex? And what locks/unlocks provide synchronization? I
>> >> see that one uses exp_mutex and another -- exp_wake_mutex.
>> >
>> > Both of them.
>> >
>> > ->exp_mutex is acquired by the task requesting the grace period, and
>> > the counter's first increment is done by that task under that mutex.
>> > This task then schedules a workqueue, which drives forward the grace
>> > period.  Upon grace-period completion, the workqueue handler does the
>> > second increment (the one that your patch addressed).  The workqueue
>> > handler then acquires ->exp_wake_mutex and wakes the task that holds
>> > ->exp_mutex (along with all other tasks waiting for this grace period),
>> > and that task releases ->exp_mutex, which allows the next grace period to
>> > start (and the first increment for that next grace period to be carried
>> > out under that lock).  The workqueue handler releases ->exp_wake_mutex
>> > after finishing its wakeups.
>>
>>
>> Then we need the following for the case when task requesting the grace
>> period does not block, right?
>>
>
> Won't be necessary I think, as the smp_mb() in rcu_seq_end() and the
> smp_mb__before_atomic() in sync_exp_work_done() already provide the
> required ordering, no?

smp_mb() is probably fine, but smp_mb__before_atomic() is release not
acquire. If we want to play that game, then I guess we also need
smp_mb__after_atomic() there. But it would be way easier to understand
what's happens there and prove that it's correct, if we use
store_release/load_acquire.