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authorPaul E. McKenney <paulmck@kernel.org>2021-06-24 18:05:52 +0200
committerDaniel Borkmann <daniel@iogearbox.net>2021-06-24 19:41:14 +0200
commit9a145c04a293933002ec288a4d6b4f370b59e4d1 (patch)
tree33bbe908aa6b5f06fd163225015dc7c784a95139 /Documentation/RCU
parentb9964ce74544ea6cbc4eabd2c89a531adf7f291d (diff)
doc: Clarify and expand RCU updaters and corresponding readers
This commit clarifies which primitives readers can use given that the corresponding updaters have made a specific choice. This commit also adds this information for the various RCU Tasks flavors. While in the area, it removes a paragraph that no longer applies in any straightforward manner. Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20210624160609.292325-3-toke@redhat.com
Diffstat (limited to 'Documentation/RCU')
-rw-r--r--Documentation/RCU/checklist.rst48
1 files changed, 27 insertions, 21 deletions
diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 1030119294d0..07f6cb8f674d 100644
--- a/Documentation/RCU/checklist.rst
+++ b/Documentation/RCU/checklist.rst
@@ -211,27 +211,33 @@ over a rather long period of time, but improvements are always welcome!
of the system, especially to real-time workloads running on
the rest of the system.
-7. As of v4.20, a given kernel implements only one RCU flavor,
- which is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y.
- If the updater uses call_rcu() or synchronize_rcu(),
- then the corresponding readers may use rcu_read_lock() and
- rcu_read_unlock(), rcu_read_lock_bh() and rcu_read_unlock_bh(),
- or any pair of primitives that disables and re-enables preemption,
- for example, rcu_read_lock_sched() and rcu_read_unlock_sched().
- If the updater uses synchronize_srcu() or call_srcu(),
- then the corresponding readers must use srcu_read_lock() and
- srcu_read_unlock(), and with the same srcu_struct. The rules for
- the expedited primitives are the same as for their non-expedited
- counterparts. Mixing things up will result in confusion and
- broken kernels, and has even resulted in an exploitable security
- issue.
-
- One exception to this rule: rcu_read_lock() and rcu_read_unlock()
- may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
- in cases where local bottom halves are already known to be
- disabled, for example, in irq or softirq context. Commenting
- such cases is a must, of course! And the jury is still out on
- whether the increased speed is worth it.
+7. As of v4.20, a given kernel implements only one RCU flavor, which
+ is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y.
+ If the updater uses call_rcu() or synchronize_rcu(), then
+ the corresponding readers may use: (1) rcu_read_lock() and
+ rcu_read_unlock(), (2) any pair of primitives that disables
+ and re-enables softirq, for example, rcu_read_lock_bh() and
+ rcu_read_unlock_bh(), or (3) any pair of primitives that disables
+ and re-enables preemption, for example, rcu_read_lock_sched() and
+ rcu_read_unlock_sched(). If the updater uses synchronize_srcu()
+ or call_srcu(), then the corresponding readers must use
+ srcu_read_lock() and srcu_read_unlock(), and with the same
+ srcu_struct. The rules for the expedited RCU grace-period-wait
+ primitives are the same as for their non-expedited counterparts.
+
+ If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(),
+ then the readers must refrain from executing voluntary
+ context switches, that is, from blocking. If the updater uses
+ call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then
+ the corresponding readers must use rcu_read_lock_trace() and
+ rcu_read_unlock_trace(). If an updater uses call_rcu_tasks_rude()
+ or synchronize_rcu_tasks_rude(), then the corresponding readers
+ must use anything that disables interrupts.
+
+ Mixing things up will result in confusion and broken kernels, and
+ has even resulted in an exploitable security issue. Therefore,
+ when using non-obvious pairs of primitives, commenting is of
+ course a must.
8. Although synchronize_rcu() is slower than is call_rcu(), it
usually results in simpler code. So, unless update performance is