diff options
Diffstat (limited to 'Documentation/RCU/UP.rst')
| -rw-r--r-- | Documentation/RCU/UP.rst | 17 |
1 files changed, 13 insertions, 4 deletions
diff --git a/Documentation/RCU/UP.rst b/Documentation/RCU/UP.rst index e26dda27430c..4060d7a2f62a 100644 --- a/Documentation/RCU/UP.rst +++ b/Documentation/RCU/UP.rst @@ -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 ------- @@ -98,7 +107,7 @@ UP systems, including PREEMPT SMP builds running on UP systems. Quick Quiz #3: Why can't synchronize_rcu() return immediately on UP systems running - preemptable RCU? + preemptible RCU? .. _answer_quick_quiz_up: @@ -134,7 +143,7 @@ Answer to Quick Quiz #2: Answer to Quick Quiz #3: Why can't synchronize_rcu() return immediately on UP systems - running preemptable RCU? + running preemptible RCU? Because some other task might have been preempted in the middle of an RCU read-side critical section. If synchronize_rcu() |