Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Sep 09, 2013 at 04:21:55PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 09, 2013 at 06:23:43AM -0700, Paul E. McKenney wrote:
> > Peter, in the general case, you are quite correct.  But this is a special
> > case where it really does work.
> > 
> > The key point here is that preemption and migration cannot move a task
> > from a CPU to which RCU is paying attention to a CPU that RCU is ignoring.
> 
> But there's no constraint placed on the migration mask (aka
> task_struct::cpus_allowed) and therefore it can move it thusly.
> 
> What you're trying to say is that by the time the task is running on
> another cpu, that cpu's state will match the state of the previous cpu,
> no?

Yep!  Might be a better way to put it as well.

> > So yes, by the time the task sees the return value from rcu_is_cpu_idle(),
> > that task might be running on some other CPU.  But that is OK, because
> > if RCU was paying attention to the old CPU, then RCU must also be paying
> > attention to the new CPU.
> 
> OK, fair enough.
> 
> > Here is an example of how this works:
> > 
> > 1.	Some task running on a CPU 0 (which RCU is paying attention to)
> > 	calls rcu_is_cpu_idle(), which disables preemption, checks the
> > 	per-CPU variable, sets ret to zero, then enables preemption.
> > 
> > 	At this point, the task is preempted by some high-priority task.
> > 
> > 2.	CPU 1 is currently idle, so RCU is -not- paying attention to it.
> > 	However, it is decided that our low-priority task should migrate
> > 	to CPU 1.
> > 
> > 3.	CPU 1 is sent an IPI, which forces this CPU out of idle.  This
> > 	causes rcu_idle_exit() to be called, which causes RCU to start
> > 	paying attention to CPU 1.
> 
> Just a nit, we typically try to avoid using IPIs to wake idle CPUs,
> doesn't change the story much though.

K, if I get to this level of detail in the comments, I will leave IPIs
out, and just say that the CPU is forced out of idle.

> > 4.	CPU 1 switches to the low-priority task, which now sees the
> > 	return value of rcu_is_cpu_idle().  Now, this return value did
> > 	in fact reflect the old state of CPU 0, and the state of CPU 0
> > 	might have changed.  (For example, the high-priority task might
> > 	have blocked, so that CPU 0 is now idle, which in turn would
> > 	mean that RCU is no longer paying attention to it, so that
> > 	if rcu_is_cpu_idle() was called right now, it would return
> > 	true rather than the false return computed in step 1 above.)
> > 
> > 5.	But that is OK.  Because of the way RCU and idle interact,
> > 	if a call from a given task to rcu_is_cpu_idle() returned false
> > 	some time in the past, a call from that same task will also
> > 	return false right now.
> > 
> > So yes, in general it is wrong to disable preemption, grab the value
> > of a per-CPU variable, re-enable preemption, and then return the result.
> > But there are a number of special cases where it is OK, and this is
> > one of them.
> 
> Right, worthy of comments though :-)

No argument there!

Now if we can agree on the naming and the exact per-CPU incantation...  ;-)

							Thanx, Paul