@@ -38,7 +38,7 @@ by having call_rcu() directly invoke its arguments only if it was called
from process context. However, this can fail in a similar manner.
Suppose that an RCU-based algorithm again scans a linked list containing
-elements A, B, and C in process contexts, but that it invokes a function
+elements A, B, and C in process context, but that it invokes a function
on each element as it is scanned. Suppose further that this function
deletes element B from the list, then passes it to call_rcu() for deferred
freeing. This may be a bit unconventional, but it is perfectly legal
@@ -59,7 +59,8 @@ Example 3: Death by Deadlock
Suppose that call_rcu() is invoked while holding a lock, and that the
callback function must acquire this same lock. In this case, if
call_rcu() were to directly invoke the callback, the result would
-be self-deadlock.
+be self-deadlock *even if* this invocation occurred from a later
+call_rcu() invocation a full grace period later.
In some cases, it would possible to restructure to code so that
the call_rcu() is delayed until after the lock is released. However,
@@ -85,6 +86,14 @@ Quick Quiz #2:
:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
+It is important to note that userspace RCU implementations *do*
+permit call_rcu() to directly invoke callbacks, but only if a full
+grace period has elapsed since those callbacks were queued. This is
+the case because some userspace environments are extremely constrained.
+Nevertheless, people writing userspace RCU implementations are strongly
+encouraged to avoid invoking callbacks from call_rcu(), thus obtaining
+the deadlock-avoidance benefits called out above.
+
Summary
-------
This commit updates UP.rst to reflect changes over the past few years, including the advent of userspace RCU libraries for constrained systems. Signed-off-by: Paul E. McKenney <paulmck@kernel.org> --- Documentation/RCU/UP.rst | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-)