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doc: Update UP.rst

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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>
This commit is contained in:
Paul E. McKenney 2022-11-04 16:45:55 -07:00
parent 0c208a7930
commit c8f2310e21
1 changed files with 11 additions and 2 deletions
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13
Documentation/RCU/UP.rst
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@ -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. from process context. However, this can fail in a similar manner.
Suppose that an RCU-based algorithm again scans a linked list containing 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 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 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 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 Suppose that call_rcu() is invoked while holding a lock, and that the
callback function must acquire this same lock. In this case, if callback function must acquire this same lock. In this case, if
call_rcu() were to directly invoke the callback, the result would 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 In some cases, it would possible to restructure to code so that
the call_rcu() is delayed until after the lock is released. However, 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>` :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 Summary
------- -------