From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> This commit adds to the poll_state_synchronize_srcu() header comment describing the issues surrounding SRCU cookie overflow/wrap for the different kernel configurations. Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@xxxxxxxxxxxxxx/ Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/srcutree.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index c5d0c03..119938d 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -1058,6 +1058,21 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu); * get_state_synchronize_srcu() or start_poll_synchronize_srcu(), and * returns @true if an SRCU grace period elapsed since the time that the * cookie was created. + * + * Because cookies are finite in size, wrapping/overflow is possible. + * This is more pronounced on 32-bit systems where cookies are 32 bits, + * where in theory wrapping could happen in about 14 hours assuming + * 25-microsecond expedited SRCU grace periods. However, a more likely + * overflow lower bound is on the order of 24 days in the case of + * one-millisecond SRCU grace periods. Of course, wrapping in a 64-bit + * system requires geologic timespans, as in more than seven million years + * even for expedited SRCU grace periods. + * + * Wrapping/overflow is much more of an issue for CONFIG_SMP=n systems + * that also have CONFIG_PREEMPTION=n, which selects Tiny SRCU. This uses + * a 16-bit cookie, which rcutorture routinely wraps in a matter of a + * few minutes. If this proves to be a problem, this counter will be + * expanded to the same size as for Tree SRCU. */ bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie) { -- 2.9.5
