From dd04140531b5d38b77ad9ff7b18117654be5bf5c Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 14 Apr 2022 06:56:35 -0700 Subject: rcu: Make polled grace-period API account for expedited grace periods Currently, this code could splat: oldstate = get_state_synchronize_rcu(); synchronize_rcu_expedited(); WARN_ON_ONCE(!poll_state_synchronize_rcu(oldstate)); This situation is counter-intuitive and user-unfriendly. After all, there really was a perfectly valid full grace period right after the call to get_state_synchronize_rcu(), so why shouldn't poll_state_synchronize_rcu() know about it? This commit therefore makes the polled grace-period API aware of expedited grace periods in addition to the normal grace periods that it is already aware of. With this change, the above code is guaranteed not to splat. Please note that the above code can still splat due to counter wrap on the one hand and situations involving partially overlapping normal/expedited grace periods on the other. On 64-bit systems, the second is of course much more likely than the first. It is possible to modify this approach to prevent overlapping grace periods from causing splats, but only at the expense of greatly increasing the probability of counter wrap, as in within milliseconds on 32-bit systems and within minutes on 64-bit systems. This commit is in preparation for polled expedited grace periods. Link: https://lore.kernel.org/all/20220121142454.1994916-1-bfoster@redhat.com/ Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing Cc: Brian Foster Cc: Dave Chinner Cc: Al Viro Cc: Ian Kent Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.h | 1 + 1 file changed, 1 insertion(+) (limited to 'kernel/rcu/tree.h') diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 9c853033f159..5634e76106c4 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -325,6 +325,7 @@ struct rcu_state { unsigned long gp_wake_seq; /* ->gp_seq at ^^^. */ unsigned long gp_seq_polled; /* GP seq for polled API. */ unsigned long gp_seq_polled_snap; /* ->gp_seq_polled at normal GP start. */ + unsigned long gp_seq_polled_exp_snap; /* ->gp_seq_polled at expedited GP start. */ /* End of fields guarded by root rcu_node's lock. */ -- cgit v1.2.3