[lttng-dev] Question about lock in synchronize_rcu implementation of URCU
Mathieu Desnoyers
mathieu.desnoyers at efficios.com
Thu Apr 28 14:38:40 UTC 2016
----- On Apr 28, 2016, at 9:47 AM, Yuxin Ren ryx at gwmail.gwu.edu wrote:
> 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?
Before we redesign urcu for RT, would it be possible to simply
use pi-mutexes (priority inheritance) instead to protect grace periods
from each other with the current urcu scheme ?
Thanks,
Mathieu
>
> 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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--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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