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| author | Paul E. McKenney <paulmck@kernel.org> | 2022-11-04 16:45:55 -0700 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@kernel.org> | 2023-01-05 11:27:56 -0800 |
| commit | c8f2310e210ceedab86f1f696d27f7b5dd87959a (patch) | |
| tree | aaa6b0f8c5ad4ef8ef97ae6068f1bea3542b8269 | |
| parent | 0c208a7930228a12a78cc2c11f6f09274872ec83 (diff) | |
doc: Update UP.rst
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>
| -rw-r--r-- | Documentation/RCU/UP.rst | 13 |
1 files changed, 11 insertions, 2 deletions
diff --git a/Documentation/RCU/UP.rst b/Documentation/RCU/UP.rst index e26dda27430c..8b20fd45f255 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 ------- |