Message-Id: <20190326230432.17963-4-paulmck@linux.ibm.com>
Date:   Tue, 26 Mar 2019 16:04:32 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 4/4] doc: Fix typos and otherwise modernize checklist.txt

This commit fixes some issues with Documentation/RCU/checklist.txt.

Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 Documentation/RCU/checklist.txt | 43 +++++++++++++++++++--------------
 1 file changed, 25 insertions(+), 18 deletions(-)

diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index fcc59fea5cd4..e98ff261a438 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -318,7 +318,7 @@ over a rather long period of time, but improvements are always welcome!
 
 11.	Any lock acquired by an RCU callback must be acquired elsewhere
 	with softirq disabled, e.g., via spin_lock_irqsave(),
-	spin_lock_bh(), etc.  Failing to disable irq on a given
+	spin_lock_bh(), etc.  Failing to disable softirq on a given
 	acquisition of that lock will result in deadlock as soon as
 	the RCU softirq handler happens to run your RCU callback while
 	interrupting that acquisition's critical section.
@@ -331,13 +331,16 @@ over a rather long period of time, but improvements are always welcome!
 	must use whatever locking or other synchronization is required
 	to safely access and/or modify that data structure.
 
-	RCU callbacks are -usually- executed on the same CPU that executed
-	the corresponding call_rcu() or call_srcu().  but are by -no-
-	means guaranteed to be.  For example, if a given CPU goes offline
-	while having an RCU callback pending, then that RCU callback
-	will execute on some surviving CPU.  (If this was not the case,
-	a self-spawning RCU callback would prevent the victim CPU from
-	ever going offline.)
+	Do not assume that RCU callbacks will be executed on the same
+	CPU that executed the corresponding call_rcu() or call_srcu().
+	For example, if a given CPU goes offline while having an RCU
+	callback pending, then that RCU callback will execute on some
+	surviving CPU.	(If this was not the case, a self-spawning RCU
+	callback would prevent the victim CPU from ever going offline.)
+	Furthermore, CPUs designated by rcu_nocbs= might well -always-
+	have their RCU callbacks executed on some other CPUs, in fact,
+	for some  real-time workloads, this is the whole point of using
+	the rcu_nocbs= kernel boot parameter.
 
 13.	Unlike other forms of RCU, it -is- permissible to block in an
 	SRCU read-side critical section (demarked by srcu_read_lock()
@@ -379,8 +382,9 @@ over a rather long period of time, but improvements are always welcome!
 	never sends IPIs to other CPUs, so it is easier on
 	real-time workloads than is synchronize_rcu_expedited().
 
-	Note that rcu_dereference() and rcu_assign_pointer() relate to
-	SRCU just as they do to other forms of RCU.
+	Note that rcu_assign_pointer() relates to SRCU just as it does to
+	other forms of RCU, but instead of rcu_dereference() you should
+	use srcu_dereference() in order to avoid lockdep splats.
 
 14.	The whole point of call_rcu(), synchronize_rcu(), and friends
 	is to wait until all pre-existing readers have finished before
@@ -400,6 +404,9 @@ over a rather long period of time, but improvements are always welcome!
 	read-side critical sections.  It is the responsibility of the
 	RCU update-side primitives to deal with this.
 
+	For SRCU readers, you can use smp_mb__after_srcu_read_unlock()
+	immediately after an srcu_read_unlock() to get a full barrier.
+
 16.	Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
 	__rcu sparse checks to validate your RCU code.	These can help
 	find problems as follows:
@@ -423,15 +430,15 @@ 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.
 
-17.	If you register a callback using call_rcu() 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-
+17.	If you register a callback using call_rcu() 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 is -not-
 	guaranteed to wait for callbacks registered on other CPUs.
+	Or even on the current CPU if that CPU recently went offline
+	and came back online.
 
 	You instead need to use one of the barrier functions:
 
-- 
2.17.1

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