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doc: Remove obsolete (non-)requirement about disabling preemption

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The Requirements.html document says "Disabling Preemption Does
Not Block Grace Periods".  However this is no longer true with
the RCU consolidation.  This commit therefore removes the obsolete
(non-)requirement entirely.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Joel Fernandes (Google) 2018-10-14 14:29:55 -07:00 committed by Paul E. McKenney
parent 93eb14201f
commit 8b9df28d7f
1 changed files with 0 additions and 50 deletions
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Documentation/RCU/Design/Requirements/Requirements.html
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@ -900,8 +900,6 @@ Except where otherwise noted, these non-guarantees were premeditated.
Grace Periods Don't Partition Read-Side Critical Sections</a>
<li> <a href="#Read-Side Critical Sections Don't Partition Grace Periods">
Read-Side Critical Sections Don't Partition Grace Periods</a>
<li> <a href="#Disabling Preemption Does Not Block Grace Periods">
Disabling Preemption Does Not Block Grace Periods</a>
</ol>
<h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3>
@ -1259,54 +1257,6 @@ of RCU grace periods.
<tr><td>&nbsp;</td></tr>
</table>
<h3><a name="Disabling Preemption Does Not Block Grace Periods">
Disabling Preemption Does Not Block Grace Periods</a></h3>
<p>
There was a time when disabling preemption on any given CPU would block
subsequent grace periods.
However, this was an accident of implementation and is not a requirement.
And in the current Linux-kernel implementation, disabling preemption
on a given CPU in fact does not block grace periods, as Oleg Nesterov
<a href="https://lkml.kernel.org/g/20150614193825.GA19582@redhat.com">demonstrated</a>.
<p>
If you need a preempt-disable region to block grace periods, you need to add
<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example
as follows:
<blockquote>
<pre>
1 preempt_disable();
2 rcu_read_lock();
3 do_something();
4 rcu_read_unlock();
5 preempt_enable();
6
7 /* Spinlocks implicitly disable preemption. */
8 spin_lock(&amp;mylock);
9 rcu_read_lock();
10 do_something();
11 rcu_read_unlock();
12 spin_unlock(&amp;mylock);
</pre>
</blockquote>
<p>
In theory, you could enter the RCU read-side critical section first,
but it is more efficient to keep the entire RCU read-side critical
section contained in the preempt-disable region as shown above.
Of course, RCU read-side critical sections that extend outside of
preempt-disable regions will work correctly, but such critical sections
can be preempted, which forces <tt>rcu_read_unlock()</tt> to do
more work.
And no, this is <i>not</i> an invitation to enclose all of your RCU
read-side critical sections within preempt-disable regions, because
doing so would degrade real-time response.
<p>
This non-requirement appeared with preemptible RCU.
<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
<p>