The following commit has been merged into the core/rcu branch of tip: Commit-ID: f19920e412fdeed1e15691bcee5b40e18b8e96ff Gitweb: https://git.kernel.org/tip/f19920e412fdeed1e15691bcee5b40e18b8e96ff Author: Paul E. McKenney <paulmck@xxxxxxxxxx> AuthorDate: Thu, 06 Aug 2020 09:40:18 -07:00 Committer: Paul E. McKenney <paulmck@xxxxxxxxxx> CommitterDate: Mon, 24 Aug 2020 18:40:25 -07:00 rcu: Always set .need_qs from __rcu_read_lock() for strict GPs The ->rcu_read_unlock_special.b.need_qs field in the task_struct structure indicates that the RCU core needs a quiscent state from the corresponding task. The __rcu_read_unlock() function checks this (via an eventual call to rcu_preempt_deferred_qs_irqrestore()), and if set reports a quiscent state immediately upon exit from the outermost RCU read-side critical section. Currently, this flag is only set when the scheduling-clock interrupt decides that the current RCU grace period is too old, as in about one full second too old. But if the kernel has been built with CONFIG_RCU_STRICT_GRACE_PERIOD=y, we clearly do not want to wait that long. This commit therefore sets the .need_qs field immediately at the start of the RCU read-side critical section from within __rcu_read_lock() in order to unconditionally enlist help from __rcu_read_unlock(). But note the additional check for rcu_state.gp_kthread, which prevents attempts to awaken RCU's grace-period kthread during early boot before there is a scheduler. Leaving off this check results in early boot hangs. So early that there is no console output. Thus, this additional check fails until such time as RCU's grace-period kthread has been created, avoiding these empty-console hangs. Reported-by Jann Horn <jannh@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/tree_plugin.h | 2 ++ 1 file changed, 2 insertions(+) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 44cf77d..668bbd2 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -376,6 +376,8 @@ void __rcu_read_lock(void) rcu_preempt_read_enter(); if (IS_ENABLED(CONFIG_PROVE_LOCKING)) WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX); + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread) + WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true); barrier(); /* critical section after entry code. */ } EXPORT_SYMBOL_GPL(__rcu_read_lock);
|