[PATCH tip/core/rcu 2/6] doc: RCU documentation update
From: Paul E. McKenney
Date: Mon Jul 24 2017 - 17:38:49 EST
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
Documentation/RCU/checklist.txt | 121 +++++++++++++++++++++++++++------------
Documentation/RCU/rcu.txt | 9 +--
Documentation/RCU/rcubarrier.txt | 5 ++
Documentation/RCU/torture.txt | 20 +++++--
Documentation/RCU/whatisRCU.txt | 5 +-
5 files changed, 111 insertions(+), 49 deletions(-)
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 6beda556faf3..49747717d905 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -23,6 +23,14 @@ over a rather long period of time, but improvements are always welcome!
Yet another exception is where the low real-time latency of RCU's
read-side primitives is critically important.
+ One final exception is where RCU readers are used to prevent
+ the ABA problem (https://en.wikipedia.org/wiki/ABA_problem)
+ for lockless updates. This does result in the mildly
+ counter-intuitive situation where rcu_read_lock() and
+ rcu_read_unlock() are used to protect updates, however, this
+ approach provides the same potential simplifications that garbage
+ collectors do.
+
1. Does the update code have proper mutual exclusion?
RCU does allow -readers- to run (almost) naked, but -writers- must
@@ -40,7 +48,9 @@ over a rather long period of time, but improvements are always welcome!
explain how this single task does not become a major bottleneck on
big multiprocessor machines (for example, if the task is updating
information relating to itself that other tasks can read, there
- by definition can be no bottleneck).
+ by definition can be no bottleneck). Note that the definition
+ of "large" has changed significantly: Eight CPUs was "large"
+ in the year 2000, but a hundred CPUs was unremarkable in 2017.
2. Do the RCU read-side critical sections make proper use of
rcu_read_lock() and friends? These primitives are needed
@@ -55,6 +65,12 @@ over a rather long period of time, but improvements are always welcome!
Disabling of preemption can serve as rcu_read_lock_sched(), but
is less readable.
+ Letting RCU-protected pointers "leak" out of an RCU read-side
+ critical section is every bid as bad as letting them leak out
+ from under a lock. Unless, of course, you have arranged some
+ other means of protection, such as a lock or a reference count
+ -before- letting them out of the RCU read-side critical section.
+
3. Does the update code tolerate concurrent accesses?
The whole point of RCU is to permit readers to run without
@@ -78,10 +94,10 @@ over a rather long period of time, but improvements are always welcome!
This works quite well, also.
- c. Make updates appear atomic to readers. For example,
+ c. Make updates appear atomic to readers. For example,
pointer updates to properly aligned fields will
appear atomic, as will individual atomic primitives.
- Sequences of perations performed under a lock will -not-
+ Sequences of operations performed under a lock will -not-
appear to be atomic to RCU readers, nor will sequences
of multiple atomic primitives.
@@ -168,8 +184,8 @@ over a rather long period of time, but improvements are always welcome!
5. If call_rcu(), or a related primitive such as call_rcu_bh(),
call_rcu_sched(), or call_srcu() is used, the callback function
- must be written to be called from softirq context. In particular,
- it cannot block.
+ will be called from softirq context. In particular, it cannot
+ block.
6. Since synchronize_rcu() can block, it cannot be called from
any sort of irq context. The same rule applies for
@@ -178,11 +194,14 @@ over a rather long period of time, but improvements are always welcome!
synchronize_sched_expedite(), and synchronize_srcu_expedited().
The expedited forms of these primitives have the same semantics
- as the non-expedited forms, but expediting is both expensive
- and unfriendly to real-time workloads. Use of the expedited
- primitives should be restricted to rare configuration-change
- operations that would not normally be undertaken while a real-time
- workload is running.
+ as the non-expedited forms, but expediting is both expensive and
+ (with the exception of synchronize_srcu_expedited()) unfriendly
+ to real-time workloads. Use of the expedited primitives should
+ be restricted to rare configuration-change operations that would
+ not normally be undertaken while a real-time workload is running.
+ However, real-time workloads can use rcupdate.rcu_normal kernel
+ boot parameter to completely disable expedited grace periods,
+ though this might have performance implications.
In particular, if you find yourself invoking one of the expedited
primitives repeatedly in a loop, please do everyone a favor:
@@ -193,11 +212,6 @@ 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.
- In addition, it is illegal to call the expedited forms from
- a CPU-hotplug notifier, or while holding a lock that is acquired
- by a CPU-hotplug notifier. Failing to observe this restriction
- will result in deadlock.
-
7. If the updater uses call_rcu() or synchronize_rcu(), then the
corresponding readers must use rcu_read_lock() and
rcu_read_unlock(). If the updater uses call_rcu_bh() or
@@ -321,7 +335,7 @@ over a rather long period of time, but improvements are always welcome!
Similarly, disabling preemption is not an acceptable substitute
for rcu_read_lock(). Code that attempts to use preemption
disabling where it should be using rcu_read_lock() will break
- in real-time kernel builds.
+ in CONFIG_PREEMPT=y kernel builds.
If you want to wait for interrupt handlers, NMI handlers, and
code under the influence of preempt_disable(), you instead
@@ -356,23 +370,22 @@ over a rather long period of time, but improvements are always welcome!
not the case, a self-spawning RCU callback would prevent the
victim CPU from ever going offline.)
-14. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
- synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
- may only be invoked from process context. Unlike other forms of
- RCU, it -is- permissible to block in an SRCU read-side critical
- section (demarked by srcu_read_lock() and srcu_read_unlock()),
- hence the "SRCU": "sleepable RCU". Please note that if you
- don't need to sleep in read-side critical sections, you should be
- using RCU rather than SRCU, because RCU is almost always faster
- and easier to use than is SRCU.
-
- Also unlike other forms of RCU, explicit initialization
- and cleanup is required via init_srcu_struct() and
- cleanup_srcu_struct(). These are passed a "struct srcu_struct"
- that defines the scope of a given SRCU domain. Once initialized,
- the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
- synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu().
- A given synchronize_srcu() waits only for SRCU read-side critical
+14. Unlike other forms of RCU, it -is- permissible to block in an
+ SRCU read-side critical section (demarked by srcu_read_lock()
+ and srcu_read_unlock()), hence the "SRCU": "sleepable RCU".
+ Please note that if you don't need to sleep in read-side critical
+ sections, you should be using RCU rather than SRCU, because RCU
+ is almost always faster and easier to use than is SRCU.
+
+ Also unlike other forms of RCU, explicit initialization and
+ cleanup is required either at build time via DEFINE_SRCU()
+ or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct()
+ and cleanup_srcu_struct(). These last two are passed a
+ "struct srcu_struct" that defines the scope of a given
+ SRCU domain. Once initialized, the srcu_struct is passed
+ to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(),
+ synchronize_srcu_expedited(), and call_srcu(). A given
+ synchronize_srcu() waits only for SRCU read-side critical
sections governed by srcu_read_lock() and srcu_read_unlock()
calls that have been passed the same srcu_struct. This property
is what makes sleeping read-side critical sections tolerable --
@@ -390,10 +403,16 @@ over a rather long period of time, but improvements are always welcome!
Therefore, SRCU should be used in preference to rw_semaphore
only in extremely read-intensive situations, or in situations
requiring SRCU's read-side deadlock immunity or low read-side
- realtime latency.
+ realtime latency. You should also consider percpu_rw_semaphore
+ when you need lightweight readers.
- Note that, rcu_assign_pointer() relates to SRCU just as it does
- to other forms of RCU.
+ SRCU's expedited primitive (synchronize_srcu_expedited())
+ never sends IPIs to other CPUs, so it is easier on
+ real-time workloads than is synchronize_rcu_expedited(),
+ synchronize_rcu_bh_expedited() or synchronize_sched_expedited().
+
+ Note that rcu_dereference() and rcu_assign_pointer() relate to
+ SRCU just as they do to other forms of RCU.
15. The whole point of call_rcu(), synchronize_rcu(), and friends
is to wait until all pre-existing readers have finished before
@@ -435,3 +454,33 @@ over a rather long period of time, but improvements are always welcome!
These debugging aids can help you find problems that are
otherwise extremely difficult to spot.
+
+18. If you register a callback using call_rcu(), call_rcu_bh(),
+ call_rcu_sched(), or call_srcu(), and pass in a function defined
+ within a loadable module, then it in necessary to wait for
+ all pending callbacks to be invoked after the last invocation
+ and before unloading that module. Note that it is absolutely
+ -not- sufficient to wait for a grace period! The current (say)
+ synchronize_rcu() implementation waits only for all previous
+ callbacks registered on the CPU that synchronize_rcu() is running
+ on, but it is -not- guaranteed to wait for callbacks registered
+ on other CPUs.
+
+ You instead need to use one of the barrier functions:
+
+ o call_rcu() -> rcu_barrier()
+ o call_rcu_bh() -> rcu_barrier_bh()
+ o call_rcu_sched() -> rcu_barrier_sched()
+ o call_srcu() -> srcu_barrier()
+
+ However, these barrier functions are absolutely -not- guaranteed
+ to wait for a grace period. In fact, if there are no call_rcu()
+ callbacks waiting anywhere in the system, rcu_barrier() is within
+ its rights to return immediately.
+
+ So if you need to wait for both an RCU grace period and for
+ all pre-existing call_rcu() callbacks, you will need to execute
+ both rcu_barrier() and synchronize_rcu(), if necessary, using
+ something like workqueues to to execute them concurrently.
+
+ See rcubarrier.txt for more information.
diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt
index 745f429fda79..7d4ae110c2c9 100644
--- a/Documentation/RCU/rcu.txt
+++ b/Documentation/RCU/rcu.txt
@@ -76,15 +76,12 @@ o I hear that RCU is patented? What is with that?
Of these, one was allowed to lapse by the assignee, and the
others have been contributed to the Linux kernel under GPL.
There are now also LGPL implementations of user-level RCU
- available (http://lttng.org/?q=node/18).
+ available (http://liburcu.org/).
o I hear that RCU needs work in order to support realtime kernels?
- This work is largely completed. Realtime-friendly RCU can be
- enabled via the CONFIG_PREEMPT_RCU kernel configuration
- parameter. However, work is in progress for enabling priority
- boosting of preempted RCU read-side critical sections. This is
- needed if you have CPU-bound realtime threads.
+ Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU
+ kernel configuration parameter.
o Where can I find more information on RCU?
diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt
index b10cfe711e68..5d7759071a3e 100644
--- a/Documentation/RCU/rcubarrier.txt
+++ b/Documentation/RCU/rcubarrier.txt
@@ -263,6 +263,11 @@ Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
are delayed for a full grace period? Couldn't this result in
rcu_barrier() returning prematurely?
+The current rcu_barrier() implementation is more complex, due to the need
+to avoid disturbing idle CPUs (especially on battery-powered systems)
+and the need to minimally disturb non-idle CPUs in real-time systems.
+However, the code above illustrates the concepts.
+
rcu_barrier() Summary
diff --git a/Documentation/RCU/torture.txt b/Documentation/RCU/torture.txt
index 278f6a9383b6..55918b54808b 100644
--- a/Documentation/RCU/torture.txt
+++ b/Documentation/RCU/torture.txt
@@ -276,15 +276,17 @@ o "Free-Block Circulation": Shows the number of torture structures
somehow gets incremented farther than it should.
Different implementations of RCU can provide implementation-specific
-additional information. For example, SRCU provides the following
+additional information. For example, Tree SRCU provides the following
additional line:
- srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1)
+ srcud-torture: Tree SRCU per-CPU(idx=0): 0(35,-21) 1(-4,24) 2(1,1) 3(-26,20) 4(28,-47) 5(-9,4) 6(-10,14) 7(-14,11) T(1,6)
-This line shows the per-CPU counter state. The numbers in parentheses are
-the values of the "old" and "current" counters for the corresponding CPU.
-The "idx" value maps the "old" and "current" values to the underlying
-array, and is useful for debugging.
+This line shows the per-CPU counter state, in this case for Tree SRCU
+using a dynamically allocated srcu_struct (hence "srcud-" rather than
+"srcu-"). The numbers in parentheses are the values of the "old" and
+"current" counters for the corresponding CPU. The "idx" value maps the
+"old" and "current" values to the underlying array, and is useful for
+debugging. The final "T" entry contains the totals of the counters.
USAGE
@@ -304,3 +306,9 @@ checked for such errors. The "rmmod" command forces a "SUCCESS",
"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed. The first
two are self-explanatory, while the last indicates that while there
were no RCU failures, CPU-hotplug problems were detected.
+
+However, the tools/testing/selftests/rcutorture/bin/kvm.sh script
+provides better automation, including automatic failure analysis.
+It assumes a qemu/kvm-enabled platform, and runs guest OSes out of initrd.
+See tools/testing/selftests/rcutorture/doc/initrd.txt for instructions
+on setting up such an initrd.
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 8ed6c9f6133c..df62466da4e0 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -890,6 +890,8 @@ SRCU: Critical sections Grace period Barrier
srcu_read_lock_held
SRCU: Initialization/cleanup
+ DEFINE_SRCU
+ DEFINE_STATIC_SRCU
init_srcu_struct
cleanup_srcu_struct
@@ -913,7 +915,8 @@ a. Will readers need to block? If so, you need SRCU.
b. What about the -rt patchset? If readers would need to block
in an non-rt kernel, you need SRCU. If readers would block
in a -rt kernel, but not in a non-rt kernel, SRCU is not
- necessary.
+ necessary. (The -rt patchset turns spinlocks into sleeplocks,
+ hence this distinction.)
c. Do you need to treat NMI handlers, hardirq handlers,
and code segments with preemption disabled (whether
--
2.5.2