Commit 4e7ccfae authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

srcu: Add comment explaining cookie overflow/wrap 

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@moria.home.lan/


Reported-by: default avatarKent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent ee7f4a87

kernel/rcu/srcutree.c

+15 −0
Original line number Diff line number Diff line
@@ -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)

{