linux-kernel - [tip: core/rcu] doc: Update list_for_each_entry

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Message-ID: <157252290045.29376.4451520799728016004.tip-bot2@tip-bot2>
Date:   Thu, 31 Oct 2019 11:55:00 -0000
From:   "tip-bot2 for Joel Fernandes (Google)" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org
Subject: [tip: core/rcu] doc: Update list_for_each_entry_rcu() documentation

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     45271064e1caccd1ab004bf747f3ff6ef4e4ea8f
Gitweb:        https://git.kernel.org/tip/45271064e1caccd1ab004bf747f3ff6ef4e4ea8f
Author:        Joel Fernandes (Google) <joel@...lfernandes.org>
AuthorDate:    Sun, 11 Aug 2019 18:11:10 -04:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Tue, 29 Oct 2019 02:48:28 -07:00

doc: Update list_for_each_entry_rcu() documentation

This commit updates the documentation with information about
usage of lockdep with list_for_each_entry_rcu().

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
[ paulmck: Wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 Documentation/RCU/lockdep.txt   | 18 ++++++++++++++----
 Documentation/RCU/whatisRCU.txt | 10 +++++++++-
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index da51d30..89db949 100644
--- a/Documentation/RCU/lockdep.txt
+++ b/Documentation/RCU/lockdep.txt
@@ -96,7 +96,17 @@ other flavors of rcu_dereference().  On the other hand, it is illegal
 to use rcu_dereference_protected() if either the RCU-protected pointer
 or the RCU-protected data that it points to can change concurrently.
 
-There are currently only "universal" versions of the rcu_assign_pointer()
-and RCU list-/tree-traversal primitives, which do not (yet) check for
-being in an RCU read-side critical section.  In the future, separate
-versions of these primitives might be created.
+Like rcu_dereference(), when lockdep is enabled, RCU list and hlist
+traversal primitives check for being called from within an RCU read-side
+critical section.  However, a lockdep expression can be passed to them
+as a additional optional argument.  With this lockdep expression, these
+traversal primitives will complain only if the lockdep expression is
+false and they are called from outside any RCU read-side critical section.
+
+For example, the workqueue for_each_pwq() macro is intended to be used
+either within an RCU read-side critical section or with wq->mutex held.
+It is thus implemented as follows:
+
+	#define for_each_pwq(pwq, wq)
+		list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node,
+					lock_is_held(&(wq->mutex).dep_map))
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 17f4831..58ba05c 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -290,7 +290,7 @@ rcu_dereference()
 	at any time, including immediately after the rcu_dereference().
 	And, again like rcu_assign_pointer(), rcu_dereference() is
 	typically used indirectly, via the _rcu list-manipulation
-	primitives, such as list_for_each_entry_rcu().
+	primitives, such as list_for_each_entry_rcu() [2].
 
 	[1] The variant rcu_dereference_protected() can be used outside
 	of an RCU read-side critical section as long as the usage is
@@ -305,6 +305,14 @@ rcu_dereference()
 	a lockdep splat is emitted.  See Documentation/RCU/Design/Requirements/Requirements.rst
 	and the API's code comments for more details and example usage.
 
+	[2] If the list_for_each_entry_rcu() instance might be used by
+	update-side code as well as by RCU readers, then an additional
+	lockdep expression can be added to its list of arguments.
+	For example, given an additional "lock_is_held(&mylock)" argument,
+	the RCU lockdep code would complain only if this instance was
+	invoked outside of an RCU read-side critical section and without
+	the protection of mylock.
+
 The following diagram shows how each API communicates among the
 reader, updater, and reclaimer.