Commit 2a5b0c84 authored by Jiunn Chang's avatar Jiunn Chang Committed by Jonathan Corbet

Documentation: RCU: Convert RCU UP systems to reST

RCU UP systems reST markup.
Reviewed-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: default avatarJiunn Chang <c0d1n61at3@gmail.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 9422dc24
RCU on Uniprocessor Systems
.. _up_doc:
RCU on Uniprocessor Systems
===========================
A common misconception is that, on UP systems, the call_rcu() primitive
may immediately invoke its function. The basis of this misconception
is that since there is only one CPU, it should not be necessary to
wait for anything else to get done, since there are no other CPUs for
anything else to be happening on. Although this approach will -sort- -of-
anything else to be happening on. Although this approach will *sort of*
work a surprising amount of the time, it is a very bad idea in general.
This document presents three examples that demonstrate exactly how bad
an idea this is.
Example 1: softirq Suicide
--------------------------
Suppose that an RCU-based algorithm scans a linked list containing
elements A, B, and C in process context, and can delete elements from
......@@ -28,8 +30,8 @@ your kernel.
This same problem can occur if call_rcu() is invoked from a hardware
interrupt handler.
Example 2: Function-Call Fatality
---------------------------------
Of course, one could avert the suicide described in the preceding example
by having call_rcu() directly invoke its arguments only if it was called
......@@ -46,11 +48,13 @@ its arguments would cause it to fail to make the fundamental guarantee
underlying RCU, namely that call_rcu() defers invoking its arguments until
all RCU read-side critical sections currently executing have completed.
Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in
this case?
Quick Quiz #1:
Why is it *not* legal to invoke synchronize_rcu() in this case?
:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
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
......@@ -76,25 +80,30 @@ there are cases where this can be quite ugly:
If call_rcu() directly invokes the callback, painful locking restrictions
or API changes would be required.
Quick Quiz #2: What locking restriction must RCU callbacks respect?
Quick Quiz #2:
What locking restriction must RCU callbacks respect?
:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
Summary
-------
Permitting call_rcu() to immediately invoke its arguments breaks RCU,
even on a UP system. So do not do it! Even on a UP system, the RCU
infrastructure -must- respect grace periods, and -must- invoke callbacks
infrastructure *must* respect grace periods, and *must* invoke callbacks
from a known environment in which no locks are held.
Note that it -is- safe for synchronize_rcu() to return immediately on
UP systems, including !PREEMPT SMP builds running on UP systems.
Note that it *is* safe for synchronize_rcu() to return immediately on
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?
Quick Quiz #3:
Why can't synchronize_rcu() return immediately on UP systems running
preemptable RCU?
.. _answer_quick_quiz_up:
Answer to Quick Quiz #1:
Why is it -not- legal to invoke synchronize_rcu() in this case?
Why is it *not* legal to invoke synchronize_rcu() in this case?
Because the calling function is scanning an RCU-protected linked
list, and is therefore within an RCU read-side critical section.
......@@ -119,7 +128,7 @@ Answer to Quick Quiz #2:
This restriction might seem gratuitous, since very few RCU
callbacks acquire locks directly. However, a great many RCU
callbacks do acquire locks -indirectly-, for example, via
callbacks do acquire locks *indirectly*, for example, via
the kfree() primitive.
Answer to Quick Quiz #3:
......
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