From 8b9df28d7f2e50dc1be758e98dad61ec77d6f6b5 Mon Sep 17 00:00:00 2001 From: "Joel Fernandes (Google)" Date: Sun, 14 Oct 2018 14:29:55 -0700 Subject: [PATCH] doc: Remove obsolete (non-)requirement about disabling preemption 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) Signed-off-by: Paul E. McKenney --- .../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
  • Read-Side Critical Sections Don't Partition Grace Periods -
  • - Disabling Preemption Does Not Block Grace Periods

    Readers Impose Minimal Ordering

    @@ -1259,54 +1257,6 @@ of RCU grace periods.   -

    -Disabling Preemption Does Not Block Grace Periods

    - -

    -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 -demonstrated. - -

    -If you need a preempt-disable region to block grace periods, you need to add -rcu_read_lock() and rcu_read_unlock(), for example -as follows: - -

    -
    - 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);
-
    -
    - -

    -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 rcu_read_unlock() to do -more work. -And no, this is not an invitation to enclose all of your RCU -read-side critical sections within preempt-disable regions, because -doing so would degrade real-time response. - -

    -This non-requirement appeared with preemptible RCU. -

    Parallelism Facts of Life