[PATCH tip/core/rcu 1/6] rcu: Update documentation to cover call_srcu() and srcu_barrier().

Paul E. McKenney
2012-06-15T19:01:10Z

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

The advent of call_srcu() and srcu_barrier() obsoleted some of the
documentation, so this commit brings that up to date.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/checklist.txt | 39 +++++++++++++++++++------------------
Documentation/RCU/rcubarrier.txt | 6 +++-
Documentation/RCU/torture.txt | 9 ++++++++
Documentation/RCU/whatisRCU.txt | 6 ++--
4 files changed, 36 insertions(+), 24 deletions(-)

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 5c8d749..fc103d7 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -162,9 +162,9 @@ over a rather long period of time, but improvements are always welcome!
when publicizing a pointer to a structure that can
be traversed by an RCU read-side critical section.

-5. If call_rcu(), or a related primitive such as call_rcu_bh() or
- call_rcu_sched(), is used, the callback function must be
- written to be called from softirq context. In particular,
+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.

6. Since synchronize_rcu() can block, it cannot be called from
@@ -202,11 +202,12 @@ over a rather long period of time, but improvements are always welcome!
updater uses call_rcu_sched() or synchronize_sched(), then
the corresponding readers must disable preemption, possibly
by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
- If the updater uses synchronize_srcu(), the 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.
+ If the updater uses synchronize_srcu() or call_srcu(),
+ the 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.

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()
@@ -333,14 +334,14 @@ over a rather long period of time, but improvements are always welcome!
victim CPU from ever going offline.)

14. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
- synchronize_srcu(), and synchronize_srcu_expedited()) 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.
+ 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.

If you need to enter your read-side critical section in a
hardirq or exception handler, and then exit that same read-side
@@ -353,8 +354,8 @@ over a rather long period of time, but improvements are always welcome!
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(), and synchronize_srcu_expedited(). A given
- synchronize_srcu() waits only for SRCU read-side critical
+ 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 --
@@ -374,7 +375,7 @@ over a rather long period of time, but improvements are always welcome!
requiring SRCU's read-side deadlock immunity or low read-side
realtime latency.

- Note that, rcu_assign_pointer() relates to SRCU just as they do
+ Note that, rcu_assign_pointer() relates to SRCU just as it does
to other forms of RCU.

15. The whole point of call_rcu(), synchronize_rcu(), and friends
diff --git a/Documentation/RCU/rcubarrier.txt b/Documentation/RCU/rcubarrier.txt
index e439a0e..a511fe6 100644
--- a/Documentation/RCU/rcubarrier.txt
+++ b/Documentation/RCU/rcubarrier.txt
@@ -261,8 +261,10 @@ Answers to Quick Quizzes

Quick Quiz #1: Why is there no srcu_barrier()?

-Answer: Since there is no call_srcu(), there can be no outstanding SRCU
- callbacks. Therefore, there is no need to wait for them.
+Answer: There really is an srcu_barrier() now that there is a call_srcu().
+ Before call_srcu(), there were no SRCU callbacks, and there was
+ therefore no need to wait for them, and therefore there was
+ no srcu_barrier().

Quick Quiz #2: Is there any other situation where rcu_barrier() might
be required?
diff --git a/Documentation/RCU/torture.txt b/Documentation/RCU/torture.txt
index 4ddf391..7dce8a1 100644
--- a/Documentation/RCU/torture.txt
+++ b/Documentation/RCU/torture.txt
@@ -174,11 +174,20 @@ torture_type The type of RCU to test, with string values as follows:
and synchronize_rcu_bh_expedited().

"srcu": srcu_read_lock(), srcu_read_unlock() and
+ call_srcu().
+
+ "srcu_sync": srcu_read_lock(), srcu_read_unlock() and
synchronize_srcu().

"srcu_expedited": srcu_read_lock(), srcu_read_unlock() and
synchronize_srcu_expedited().

+ "srcu_raw": srcu_read_lock_raw(), srcu_read_unlock_raw(),
+ and call_srcu().
+
+ "srcu_raw_sync": srcu_read_lock_raw(), srcu_read_unlock_raw(),
+ and synchronize_srcu().
+
"sched": preempt_disable(), preempt_enable(), and
call_rcu_sched().

diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 6bbe8dc..69ee188 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -833,9 +833,9 @@ sched: Critical sections Grace period Barrier

SRCU: Critical sections Grace period Barrier

- srcu_read_lock synchronize_srcu N/A
- srcu_read_unlock synchronize_srcu_expedited
- srcu_read_lock_raw
+ srcu_read_lock synchronize_srcu srcu_barrier
+ srcu_read_unlock call_srcu
+ srcu_read_lock_raw synchronize_srcu_expedited
srcu_read_unlock_raw
srcu_dereference

--
1.7.8