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

[Joel Fernandes <joelagnelf@nvidia.com>]

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.

thanks,

 - Joel

> 
> 							Thanx, Paul
> 
>> ---
>> v1->v2: Resend with Neeraj review tag. Rebased on rcu/dev.
>>
>>  kernel/rcu/srcutree.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>> index d2a694944553..591371d62e89 100644
>> --- a/kernel/rcu/srcutree.c
>> +++ b/kernel/rcu/srcutree.c
>> @@ -1589,7 +1589,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
>>  bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
>>  {
>>  	if (cookie != SRCU_GET_STATE_COMPLETED &&
>> -	    !rcu_seq_done(&ssp->srcu_sup->srcu_gp_seq, cookie))
>> +	    !rcu_seq_done_exact(&ssp->srcu_sup->srcu_gp_seq, cookie))
>>  		return false;
>>  	// Ensure that the end of the SRCU grace period happens before
>>  	// any subsequent code that the caller might execute.
>> -- 
>> 2.34.1
>>