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@xxxxxxxxxxxxxxxxx> 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> </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@xxxxxxxxxx";>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(&mylock); > - 9 rcu_read_lock(); > -10 do_something(); > -11 rcu_read_unlock(); > -12 spin_unlock(&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 >
