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doc: Update list_for_each_entry_rcu() documentation

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

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Joel Fernandes (Google) 2019-08-11 18:11:10 -04:00 committed by Paul E. McKenney
parent 71cb46ae46
commit 45271064e1
2 changed files with 23 additions and 5 deletions
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18
Documentation/RCU/lockdep.txt
View File

@ -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))

10
Documentation/RCU/whatisRCU.txt
View File

@ -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.