[lttng-dev] Question about lock in synchronize_rcu implementation of URCU
Yuxin Ren
ryx at gwmail.gwu.edu
Thu Apr 28 13:47:23 UTC 2016
Hi Boqun and Paul,
Thank you so much for your help.
I found one reason to use that lock.
In the slow path, a thread will move all waiters to a local queue.
https://github.com/urcu/userspace-rcu/blob/master/urcu.c#L406
After this, following thread can also perform grace period, as the
global waiter queue is empty.
Thus the rcu_gp_lock is used to ensure mutual exclusion.
However, from real time aspect, such blocking will cause priority
inversion: higher priority writer can be blocked by low priority
writer.
Is there a way to modify the code to allow multiple threads to perform
grace period concurrently?
Thanks again!!
Yuxin
On Thu, Apr 28, 2016 at 8:44 AM, Boqun Feng <boqun.feng at gmail.com> wrote:
> Hi Paul and Yuxin,
>
> On Wed, Apr 27, 2016 at 09:23:27PM -0700, Paul E. McKenney wrote:
>> Try building without it and see what happens when you run the tests.
>>
>
> I've run a 'regtest' with the following modification out of curiousity:
>
> diff --git a/urcu.c b/urcu.c
> index a5568bdbd075..9dc3c9feae56 100644
> --- a/urcu.c
> +++ b/urcu.c
> @@ -398,8 +398,6 @@ void synchronize_rcu(void)
> /* We won't need to wake ourself up */
> urcu_wait_set_state(&wait, URCU_WAIT_RUNNING);
>
> - mutex_lock(&rcu_gp_lock);
> -
> /*
> * Move all waiters into our local queue.
> */
> @@ -480,7 +478,6 @@ void synchronize_rcu(void)
> smp_mb_master();
> out:
> mutex_unlock(&rcu_registry_lock);
> - mutex_unlock(&rcu_gp_lock);
>
> /*
> * Wakeup waiters only after we have completed the grace period
>
>
> And guess what, the result of the test was:
>
> Test Summary Report
> -------------------
> ./run-urcu-tests.sh 1 (Wstat: 0 Tests: 979 Failed: 18)
> Failed tests: 30, 45, 60, 75, 90, 103, 105, 120, 135
> 150, 165, 180, 195, 210, 225, 240, 253
> 255
> Files=2, Tests=996, 1039 wallclock secs ( 0.55 usr 0.04 sys + 8981.02 cusr 597.64 csys = 9579.25 CPU)
> Result: FAIL
>
> And test case 30 for example is something like:
>
> tests/benchmark/test_urcu_mb <nreaders> <nwriters> 1 -d 0 -b 32768
>
> and it failed because:
>
> lt-test_urcu_mb: test_urcu.c:183: thr_reader: Assertion `*local_ptr == 8' failed.
> zsh: abort (core dumped) ~/userspace-rcu/tests/benchmark/test_urcu_mb 4 4 1 -d 0 -b 32768
>
> So I think what was going on here was:
>
> CPU 0 (reader) CPU 1 (writer) CPU 2 (writer)
> =================== ==================== ======================
> rcu_read_lock(); new = malloc(sizeof(int));
> local_ptr = rcu_dereference(test_rcu_pointer); // local_ptr == old *new = 8;
> old = rcu_xchg_pointer(&test_rcu_pointer, new);
> synchronize_rcu():
> urcu_wait_add(); // return 0
> urcu_move_waiters(); // @gp_waiters is empty,
> // the next urcu_wait_add() will return 0
>
> synchronize_rcu():
> urcu_wait_add(); // return 0
>
> mutex_lock(&rcu_register_lock);
> wait_for_readers(); // remove registered reader from @registery,
> // release rcu_register_lock and wait via poll()
>
> mutex_lock(&rcu_registry_lock);
> wait_for_readers(); // @regsitery is empty! we are so lucky
> return;
>
> if (old)
> free(old)
> rcu_read_unlock();
> assert(*local_ptr==8); // but local_ptr(i.e. old) is already freed.
>
>
> So the point is there could be two writers calling synchronize_rcu() but
> not returning early(both of them enter the slow path to perform a grace
> period), so the rcu_gp_lock is necessary in this case.
>
> (Cc Mathieu)
>
> But this is only my understanding and I'm learning the URCU code too ;-)
>
> Regards,
> Boqun
>
>
>> Might well be that it is unnecessary, but I will defer to Mathieu
>> on that point.
>>
>> Thanx, Paul
>>
>> On Wed, Apr 27, 2016 at 10:18:04PM -0400, Yuxin Ren wrote:
>> > As they don't currently perform grace period, why do we use the rcu_gp_lock?
>> >
>> > Thank you.
>> > Yuxin
>> >
>> > On Wed, Apr 27, 2016 at 10:08 PM, Paul E. McKenney
>> > <paulmck at linux.vnet.ibm.com> wrote:
>> > > On Wed, Apr 27, 2016 at 09:34:16PM -0400, Yuxin Ren wrote:
>> > >> Hi,
>> > >>
>> > >> I am learning the URCU code.
>> > >>
>> > >> Why do we need rcu_gp_lock in synchronize_rcu?
>> > >> https://github.com/urcu/userspace-rcu/blob/master/urcu.c#L401
>> > >>
>> > >> In the comment, it says this lock ensures mutual exclusion between
>> > >> threads calling synchronize_rcu().
>> > >> But only the first thread added to waiter queue can proceed to detect
>> > >> grace period.
>> > >> How can multiple threads currently perform the grace thread?
>> > >
>> > > They don't concurrently perform grace periods, and it would be wasteful
>> > > for them to do so. Instead, the first one performs the grace period,
>> > > and all that were waiting at the time it started get the benefit of that
>> > > same grace period.
>> > >
>> > > Any that arrived after the first grace period performs the first
>> > > grace period are served by whichever of them performs the second
>> > > grace period.
>> > >
>> > > Thanx, Paul
>> > >
>> >
>>
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