linux-kernel - Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption

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Message-Id: <20181014231731.GN2674@linux.ibm.com>
Date:   Sun, 14 Oct 2018 16:17:31 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about
 disabling preemption

On Sun, Oct 14, 2018 at 02:29:55PM -0700, Joel Fernandes (Google) wrote:
> The Requirements.html document says "Disabling Preemption Does Not Block
> Grace Periods". However this is no longer true with the RCU
> consolidation. Lets remove the obsolete (non-)requirement entirely.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Good catch, queued, thank you!

							Thanx, Paul

> ---
>  .../RCU/Design/Requirements/Requirements.html | 50 -------------------
>  1 file changed, 50 deletions(-)
> 
> diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
> index 7efc1c1da7af..4fae55056c1d 100644
> --- a/Documentation/RCU/Design/Requirements/Requirements.html
> +++ b/Documentation/RCU/Design/Requirements/Requirements.html
> @@ -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>
> -- 
> 2.19.0.605.g01d371f741-goog
>