On Wed, Feb 19, 2025 at 06:45:07PM -0500, Joel Fernandes wrote: > > > 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? Reviewed-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
