+Prior to v4.4, it was only necessary to disable preemption across +RCU read-side critical sections that acquired scheduler locks. +In v4.4, expedited grace periods started using IPIs, and these +IPIs could force a rcu_read_unlock() to take the slowpath. +Therefore, this expedited-grace-period change required disabling of +interrupts, not just preemption. +
For RCU's part, the preemptible-RCU rcu_read_unlock() implementation must be written carefully to avoid similar deadlocks. diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx index 2d0cd90987f6e..3a97ba490c42b 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx @@ -2124,7 +2124,7 @@ the runqueue locks and the priority-inheritance locks in the middle of an outermost RCU read-side critical section unless either (1) it releases them before exiting that same RCU read-side critical section, or -(2) preemption is disabled across +(2) interrupts are disabled across that entire RCU read-side critical section. This same prohibition also applies (recursively!) to any lock that is acquired while holding any lock to which this prohibition applies. @@ -2132,6 +2132,14 @@ Adhering to this rule prevents preemptible RCU from invoking rcu_read_unlock_special() while either runqueue or priority-inheritance locks are held, thus avoiding deadlock. +
+Prior to v4.4, it was only necessary to disable preemption across +RCU read-side critical sections that acquired scheduler locks. +In v4.4, expedited grace periods started using IPIs, and these +IPIs could force a rcu_read_unlock() to take the slowpath. +Therefore, this expedited-grace-period change required disabling of +interrupts, not just preemption. +
For RCU's part, the preemptible-RCU rcu_read_unlock() implementation must be written carefully to avoid similar deadlocks.