On Tue, Jun 25, 2019 at 01:26:24AM -0500, Jiunn Chang wrote: > RCU UP systems reST markup. > > Signed-off-by: Jiunn Chang <c0d1n61at3@xxxxxxxxx> > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > Documentation/RCU/UP.txt | 37 +++++++++++++++++++++++-------------- > 1 file changed, 23 insertions(+), 14 deletions(-) > > diff --git a/Documentation/RCU/UP.txt b/Documentation/RCU/UP.txt > index 53bde717017b..67715a47ae89 100644 > --- a/Documentation/RCU/UP.txt > +++ b/Documentation/RCU/UP.txt > @@ -1,17 +1,19 @@ > -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: As long as you are in the area, the answer is overly constraining. The locking primitives could use either _irq suffixes (as stated) or _bh suffixes. This is obviously not your fault, but please feel free to fix this with an additional patch. Thanx, Paul > 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: > -- > 2.22.0 >
