diff options
author | Paul E. McKenney <paulmck@kernel.org> | 2020-11-15 20:33:38 -0800 |
---|---|---|
committer | Paul E. McKenney <paulmck@kernel.org> | 2021-01-04 13:53:39 -0800 |
commit | 4e7ccfae52b39aeee93ed39d4184d50ea201fbef (patch) | |
tree | bc89d83fd6df7810f3a8f535c77c260af33850b8 /kernel | |
parent | ee7f4a87a18cd3bb141b38e2ef0c3e53253cdf63 (diff) | |
download | linux-stable-4e7ccfae52b39aeee93ed39d4184d50ea201fbef.tar.gz linux-stable-4e7ccfae52b39aeee93ed39d4184d50ea201fbef.tar.bz2 linux-stable-4e7ccfae52b39aeee93ed39d4184d50ea201fbef.zip |
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: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'kernel')
-rw-r--r-- | kernel/rcu/srcutree.c | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index c5d0c036fab5..119938d381e2 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) { |