Re: [PATCH] rcu: Merge rcu_seq_done_exact() logic into rcu_seq_done()

[Joel Fernandes <joel@joelfernandes.org>]

> On Jan 28, 2025, at 9:21 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> On Tue, Jan 28, 2025 at 09:13:45PM -0500, Joel Fernandes wrote:
>>> On Tue, Jan 28, 2025 at 8:47 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>>> 
>>> On Tue, Jan 28, 2025 at 08:38:57PM -0500, Joel Fernandes wrote:
>>>> On Tue, Jan 28, 2025 at 8:33 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>>>>> 
>>>>> On Tue, Jan 28, 2025 at 08:22:48PM -0500, Joel Fernandes wrote:
>>>>>> On Mon, Jan 27, 2025 at 07:09:34PM -0500, Joel Fernandes wrote:
>>>>>>> On Mon, Jan 27, 2025 at 7:07 PM Joel Fernandes (Google)
>>>>>>> <joel@joelfernandes.org> wrote:
>>>>>>>> 
>>>>>>>> The rcu_seq_done() API has a large "false-negative" windows of size
>>>>>>>> ULONG_MAX/2, where after wrap around, it is possible that it will think
>>>>>>>> that a GP has not completed if a wrap around happens and the delta is
>>>>>>>> large.
>>>>>>>> 
>>>>>>>> rcu_seq_done_exact() is more accurate avoiding this wrap around issue,
>>>>>>>> by making the window of false-negativity by only 3 GPs. Use this logic
>>>>>>>> for rcu_seq_done() which is a nice negative code delta and could
>>>>>>>> potentially avoid issues in the future where rcu_seq_done() was
>>>>>>>> reporting false-negatives for too long.
>>>>>>>> 
>>>>>>>> rcutorture runs of all scenarios for 15 minutes passed. Code inspection
>>>>>>>> was done of all users to convince the change would work.
>>>>>>>> 
>>>>>>>> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>>>>>>> 
>>>>>>> I am leaving a 60 minute overnight run of all scenarios on my personal
>>>>>>> server for further testing.
>>>>>> 
>>>>>> The run passed, details below:
>>>>>> 
>>>>>> --- Mon Jan 27 11:49:49 PM EST 2025 Test summary:
>>>>>> Results directory:
>>>>>> tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 60
>>>>>> RUDE01 ------- 14309 GPs (3.97472/s) [tasks-rude: g57884 f0x0 total-gps=57880] n_max_cbs: 0
>>>>>> SRCU-L ------- 34121 GPs (9.47806/s) [srcu: g316564 f0x0 total-gps=79242] n_max_cbs: 150000
>>>>>> SRCU-N ------- 151316 GPs (42.0322/s) [srcu: g1840064 f0x0 total-gps=460117] n_max_cbs: 150000
>>>>>> SRCU-P ------- 35189 GPs (9.77472/s) [srcud: g320792 f0x0 total-gps=80299] n_max_cbs: 150000
>>>>>> SRCU-T ------- 389034 GPs (108.065/s) [srcu: g4142406 f0x0 total-gps=1035602] n_max_cbs: 50000
>>>>>> SRCU-U ------- 376267 GPs (104.519/s) [srcud: g3953834 f0x0 total-gps=988459] n_max_cbs: 50000
>>>>>> SRCU-V ------- 407633 GPs (113.231/s) [srcud: g4371704 f0x0 total-gps=1092927] n_max_cbs: 1000
>>>>>> TASKS01 ------- 11007 GPs (3.0575/s) [tasks: g57816 f0x0 total-gps=57808]
>>>>>> TASKS02 ------- 10539 GPs (2.9275/s) [tasks: g57936 f0x0 total-gps=57936]
>>>>>> TASKS03 ------- 10453 GPs (2.90361/s) [tasks: g57508 f0x0 total-gps=57508]
>>>>>> TINY01 ------- 511634 GPs (142.121/s) [rcu: g0 f0x0 total-gps=0] n_max_cbs: 57078
>>>>>> TINY02 ------- 541799 GPs (150.5/s) [rcu: g0 f0x0 total-gps=0] n_max_cbs: 2619
>>>>>> TRACE01 ------- 7299 GPs (2.0275/s) [tasks-tracing: g45844 f0x0 total-gps=45844] n_max_cbs: 50000
>>>>>> TRACE02 ------- 101265 GPs (28.1292/s) [tasks-tracing: g305464 f0x0 total-gps=305456] n_max_cbs: 100000
>>>>>> TREE01 ------- 97989 GPs (27.2192/s) [rcu: g479473 f0x0 total-gps=120151]
>>>>>> TREE02 ------- 202908 GPs (56.3633/s) [rcu: g1459509 f0x0 total-gps=365162] n_max_cbs: 1139244
>>>>>> TREE03 ------- 168901 GPs (46.9169/s) [rcu: g1764445 f0x0 total-gps=441393] n_max_cbs: 1341765
>>>>>> TREE04 ------- 148876 GPs (41.3544/s) [rcu: g951744 f0x0 total-gps=238225] n_max_cbs: 236765
>>>>>> TREE05 ------- 220092 GPs (61.1367/s) [rcu: g1234385 f0x0 total-gps=308880] n_max_cbs: 82801
>>>>>> TREE07 ------- 34678 GPs (9.63278/s) [rcu: g207257 f0x0 total-gps=52094]
>>>>>> TREE09 ------- 341706 GPs (94.9183/s) [rcu: g7693569 f0x0 total-gps=1923688] n_max_cbs: 1845334
>>>>>> --- Done at Mon Jan 27 11:49:55 PM EST 2025 (4:41:24) exitcode 0
>>>>> 
>>>>> Very good!
>>>>> 
>>>>> How would you go about analyzing whether this is really safe vs. getting
>>>>> just getting lucky and not having provoked an overflow?
>>>> 
>>>> I would probably add a more specific test case stressing the API, or
>>>> even a unit test of just the API by passing a range of sequences.. I
>>>> should go ahead and do that but it sounds like you feel there is an
>>>> issue with the patch? :)
>>> 
>>> 2^31 (let alone 2^63) is a very large number of grace periods, and
>>> so it is hard to test grace-period sequence-number wrap.
>>> 
>>> Not impossible, though...
>> 
>> We could test a decent number of candidate sequences to cover
>> different cases. Not ideal like bruteforcing, but...   Another idea is
>> to hardcode/assume ULONG_MAX as 16-bit in a unit test.
> 
> Or put the various sequence numbers into an unsigned short or even
> an unsigned char.
> 
> One set of use cases checks to see if a given CPU's ->gp_seq has fallen
> too far behind the current grace period, and sets a flag to alert
> that CPU.  Others rely on a false negative being functionally OK.
> 
> Or so I believe.  ;-)

Thanks, I am itching to create a visualization of all eight bit combinations and the output of both API, which will be a fun exercise however I’m missing something fundamental because as I mentioned in that 100 and 200 example, the API itself cannot distinguish between a wraparound and a legitimate delay in comparison between start and a delayed end. I need to understand this better and go through the code more. ;-/

Thanks,

- Joel


> 
>                            Thanx, Paul
> 
>> thanks,
>> 
>> - Joel
>> 
>> 
>> 
>>>> 
>>>>> 
>>>>>                                                        Thanx, Paul
>>>>> 
>>>>>> thanks,
>>>>>> 
>>>>>> - Joel
>>>>>> 
>>>>>> 
>>>>>>> 
>>>>>>> thanks,
>>>>>>> 
>>>>>>> - Joel
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>>> ---
>>>>>>>> kernel/rcu/rcu.h  | 13 ++-----------
>>>>>>>> kernel/rcu/tree.c |  6 +++---
>>>>>>>> 2 files changed, 5 insertions(+), 14 deletions(-)
>>>>>>>> 
>>>>>>>> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
>>>>>>>> index eed2951a4962..c2ca196907cb 100644
>>>>>>>> --- a/kernel/rcu/rcu.h
>>>>>>>> +++ b/kernel/rcu/rcu.h
>>>>>>>> @@ -146,19 +146,10 @@ static inline bool rcu_seq_started(unsigned long *sp, unsigned long s)
>>>>>>>> 
>>>>>>>> /*
>>>>>>>>  * Given a snapshot from rcu_seq_snap(), determine whether or not a
>>>>>>>> - * full update-side operation has occurred.
>>>>>>>> + * full update-side operation has occurred while also handling
>>>>>>>> + * wraparounds that exceed the (ULONG_MAX / 2) safety-factor/guard-band.
>>>>>>>>  */
>>>>>>>> static inline bool rcu_seq_done(unsigned long *sp, unsigned long s)
>>>>>>>> -{
>>>>>>>> -       return ULONG_CMP_GE(READ_ONCE(*sp), s);
>>>>>>>> -}
>>>>>>>> -
>>>>>>>> -/*
>>>>>>>> - * Given a snapshot from rcu_seq_snap(), determine whether or not a
>>>>>>>> - * full update-side operation has occurred, but do not allow the
>>>>>>>> - * (ULONG_MAX / 2) safety-factor/guard-band.
>>>>>>>> - */
>>>>>>>> -static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
>>>>>>>> {
>>>>>>>>        unsigned long cur_s = READ_ONCE(*sp);
>>>>>>>> 
>>>>>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>>>>>> index b77ccc55557b..835600cec9ba 100644
>>>>>>>> --- a/kernel/rcu/tree.c
>>>>>>>> +++ b/kernel/rcu/tree.c
>>>>>>>> @@ -4300,7 +4300,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_full);
>>>>>>>> bool poll_state_synchronize_rcu(unsigned long oldstate)
>>>>>>>> {
>>>>>>>>        if (oldstate == RCU_GET_STATE_COMPLETED ||
>>>>>>>> -           rcu_seq_done_exact(&rcu_state.gp_seq_polled, oldstate)) {
>>>>>>>> +           rcu_seq_done(&rcu_state.gp_seq_polled, oldstate)) {
>>>>>>>>                smp_mb(); /* Ensure GP ends before subsequent accesses. */
>>>>>>>>                return true;
>>>>>>>>        }
>>>>>>>> @@ -4347,9 +4347,9 @@ bool poll_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
>>>>>>>> 
>>>>>>>>        smp_mb(); // Order against root rcu_node structure grace-period cleanup.
>>>>>>>>        if (rgosp->rgos_norm == RCU_GET_STATE_COMPLETED ||
>>>>>>>> -           rcu_seq_done_exact(&rnp->gp_seq, rgosp->rgos_norm) ||
>>>>>>>> +           rcu_seq_done(&rnp->gp_seq, rgosp->rgos_norm) ||
>>>>>>>>            rgosp->rgos_exp == RCU_GET_STATE_COMPLETED ||
>>>>>>>> -           rcu_seq_done_exact(&rcu_state.expedited_sequence, rgosp->rgos_exp)) {
>>>>>>>> +           rcu_seq_done(&rcu_state.expedited_sequence, rgosp->rgos_exp)) {
>>>>>>>>                smp_mb(); /* Ensure GP ends before subsequent accesses. */
>>>>>>>>                return true;
>>>>>>>>        }
>>>>>>>> --
>>>>>>>> 2.34.1
>>>>>>>>