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Diffstat (limited to 'Documentation/RCU/Design/Requirements/Requirements.html')
| -rw-r--r-- | Documentation/RCU/Design/Requirements/Requirements.html | 44 |
1 files changed, 29 insertions, 15 deletions
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index f74a2233865c..9fca73e03a98 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2475,23 +2475,37 @@ for context-switch-heavy <tt>CONFIG_NO_HZ_FULL=y</tt> workloads, but there is room for further improvement. <p> -In the past, it was forbidden to disable interrupts across an -<tt>rcu_read_unlock()</tt> unless that interrupt-disabled region -of code also included the matching <tt>rcu_read_lock()</tt>. -Violating this restriction could result in deadlocks involving the -scheduler's runqueue and priority-inheritance spinlocks. -This restriction was lifted when interrupt-disabled calls to -<tt>rcu_read_unlock()</tt> started deferring the reporting of -the resulting RCU-preempt quiescent state until the end of that +It is forbidden to hold any of scheduler's runqueue or priority-inheritance +spinlocks across an <tt>rcu_read_unlock()</tt> unless interrupts have been +disabled across the entire RCU read-side critical section, that is, +up to and including the matching <tt>rcu_read_lock()</tt>. +Violating this restriction can result in deadlocks involving these +scheduler spinlocks. +There was hope that this restriction might be lifted when interrupt-disabled +calls to <tt>rcu_read_unlock()</tt> started deferring the reporting of +the resulting RCU-preempt quiescent state until the end of the corresponding interrupts-disabled region. -This deferred reporting means that the scheduler's runqueue and -priority-inheritance locks cannot be held while reporting an RCU-preempt -quiescent state, which lifts the earlier restriction, at least from -a deadlock perspective. -Unfortunately, real-time systems using RCU priority boosting may +Unfortunately, timely reporting of the corresponding quiescent state +to expedited grace periods requires a call to <tt>raise_softirq()</tt>, +which can acquire these scheduler spinlocks. +In addition, real-time systems using RCU priority boosting need this restriction to remain in effect because deferred -quiescent-state reporting also defers deboosting, which in turn -degrades real-time latencies. +quiescent-state reporting would also defer deboosting, which in turn +would degrade real-time latencies. + +<p> +In theory, if a given RCU read-side critical section could be +guaranteed to be less than one second in duration, holding a scheduler +spinlock across that critical section's <tt>rcu_read_unlock()</tt> +would require only that preemption be disabled across the entire +RCU read-side critical section, not interrupts. +Unfortunately, given the possibility of vCPU preemption, long-running +interrupts, and so on, it is not possible in practice to guarantee +that a given RCU read-side critical section will complete in less than +one second. +Therefore, as noted above, if scheduler spinlocks are held across +a given call to <tt>rcu_read_unlock()</tt>, interrupts must be +disabled across the entire RCU read-side critical section. <h3><a name="Tracing and RCU">Tracing and RCU</a></h3> |