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@xxxxxxx> >>>> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx> >>> >>> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx> >>> >>> 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
