Re: [PATCH 3/3] rcu: Comment on the extraneous delta test on rcu_seq_done_exact()

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Mar 26, 2025, at 6:37 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> On Mon, Mar 24, 2025 at 01:01:55PM -0400, Joel Fernandes wrote:
>> From: Frederic Weisbecker <frederic@kernel.org>
>> 
>> The numbers used in rcu_seq_done_exact() lack some explanation behind
>> their magic. Especially after the commit:
>> 
>>    85aad7cc4178 ("rcu: Fix get_state_synchronize_rcu_full() GP-start detection")
>> 
>> which reported a subtle issue where a new GP sequence snapshot was taken
>> on the root node state while a grace period had already been started and
>> reflected on the global state sequence but not yet on the root node
>> sequence, making a polling user waiting on a wrong already started grace
>> period that would ignore freshly online CPUs.
>> 
>> The fix involved taking the snaphot on the global state sequence and
>> waiting on the root node sequence. And since a grace period is first
>> started on the global state and only afterward reflected on the root
>> node, a snapshot taken on the global state sequence might be two full
>> grace periods ahead of the root node as in the following example:
>> 
>> rnp->gp_seq = rcu_state.gp_seq = 0
>> 
>>    CPU 0                                           CPU 1
>>    -----                                           -----
>>    // rcu_state.gp_seq = 1
>>    rcu_seq_start(&rcu_state.gp_seq)
>>                                                    // snap = 8
>>                                                    snap = rcu_seq_snap(&rcu_state.gp_seq)
>>                                                    // Two full GP differences
>>                                                    rcu_seq_done_exact(&rnp->gp_seq, snap)
>>    // rnp->gp_seq = 1
>>    WRITE_ONCE(rnp->gp_seq, rcu_state.gp_seq);
>> 
>> Add a comment about those expectations and to clarify the magic within
>> the relevant function.
>> 
>> Note that the issue arises mainly with the use of rcu_seq_done_exact()
>> which has a much tigher guardband (of 2 GPs) to ensure the false-negative
>> window of the API during wraparound is limited to just 2 GPs.
>> rcu_seq_done() does not have such strict requirements, however its large
>> false-negative window of ULONG_MAX/2 is not ideal for the polling API.
>> However, this also means care is needed to ensure the guardband is as
>> large as needed to avoid the example scenario describe above which a
>> warning added in an earlier patch does.
>> 
>> [ Comment wordsmithing by Joel ]
>> 
>> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
>> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Looks good, and I stand by my Reviewed-by.  ;-)

Thanks, I will queue this one for 6.16.

- Joel


> 
>                            Thanx, Paul
> 
>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
>> ---
>> kernel/rcu/rcu.h | 9 +++++++++
>> 1 file changed, 9 insertions(+)
>> 
>> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
>> index 5e1ee570bb27..db63f330768c 100644
>> --- a/kernel/rcu/rcu.h
>> +++ b/kernel/rcu/rcu.h
>> @@ -160,6 +160,15 @@ static inline bool rcu_seq_done(unsigned long *sp, unsigned long 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.
>> + *
>> + * The token returned by get_state_synchronize_rcu_full() is based on
>> + * rcu_state.gp_seq but it is tested in poll_state_synchronize_rcu_full()
>> + * against the root rnp->gp_seq. Since rcu_seq_start() is first called
>> + * on rcu_state.gp_seq and only later reflected on the root rnp->gp_seq,
>> + * it is possible that rcu_seq_snap(rcu_state.gp_seq) returns 2 full grace
>> + * periods ahead of the root rnp->gp_seq. To prevent false-positives with the
>> + * full polling API that a wrap around instantly completed the GP, when nothing
>> + * like that happened, adjust for the 2 GPs in the ULONG_CMP_LT().
>>  */
>> static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
>> {
>> --
>> 2.43.0
>>