Re: [PATCH v2 -rcu] srcu: Use rcu_seq_done_exact() for polling API

[Joel Fernandes <joelagnelf@nvidia.com>]

On 2/19/2025 4:07 PM, Kent Overstreet wrote:
> On Wed, Feb 19, 2025 at 08:29:47AM -0500, Joel Fernandes wrote:
>>
>>
>> On 2/19/2025 8:22 AM, Paul E. McKenney wrote:
>>> On Wed, Feb 19, 2025 at 07:43:08AM -0500, Joel Fernandes wrote:
>>>> poll_state_synchronize_srcu() uses rcu_seq_done() unlike
>>>> poll_state_synchronize_rcu() which uses rcu_seq_done_exact().
>>>>
>>>> The  rcu_seq_done_exact() makes more sense for polling API, as with
>>>> this API, there is a higher chance that there is a significant delay
>>>> between the get_state..() and poll_state..() calls since a cookie
>>>> can be stored and reused at a later time. During such a delay, if
>>>> the gp_seq counter progresses more than ULONG_MAX/2 distance, then
>>>> poll_state..() may return false for a long time unwantedly.
>>>>
>>>> Fix by using the more accurate rcu_seq_done_exact() API which is
>>>> exactly what straight RCU's polling does.
>>>>
>>>> It may make sense, as future work, to add debug code here as well, where
>>>> we compare a physical timestamp between get_state..() and poll_state()
>>>> calls and yell if significant time has past but the grace period has
>>>> still not progressed.
>>>>
>>>> Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
>>>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
>>>
>>> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
>>>
>>> But we should also run this by Kent Overstreet, given that bcachefs
>>> uses this.  Should be OK, given that bcachefs uses this API in the same
>>> way that it does poll_state_synchronize_rcu(), but still...
>>
>> Thanks Paul!  Adding Kent Overstreet to the email to raise any objections.
> 
> It sounds like rcu_done_exact() is indeed what we want - bcachefs uses
> this for determining when objects may be reclaimed (as is typical with
> rcu), so we don't want objects to be stranded a "significant time past
> the grace period".

Thanks for confirming. May I add your Reviewed-by tag as well?

> 
> Is there any additional cost? I'm not seeing rcu_done_exact() in Linus's
> tree yet. Minor additional overhead would be totally fine; we use this
> from fs/bcachefs/rcu_pending.c which doesn't call it for each object.

The additional overhead should be minimal. rcu_seq_done_exact() is the function.

thanks,

 -Joel