> On Mar 26, 2025, at 6:37 PM, Paul E. McKenney <paul...@kernel.org> wrote:
>
> On Mon, Mar 24, 2025 at 01:01:55PM -0400, Joel Fernandes wrote:
>> From: Frederic Weisbecker <frede...@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 <frede...@kernel.org>
>> Reviewed-by: Paul E. McKenney <paul...@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 <joelagn...@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
>>
