Re: BUG: using smp_processor_id() in preemptible [00000000] code:asm/8267

[Paul E. McKenney]

On Wed, Mar 27, 2013 at 03:17:32PM +0100, Peter Zijlstra wrote:
> On Wed, 2013-03-27 at 14:15 +0100, Joerg Roedel wrote:
> > What makes me wonder here is that the code is preemptible in an
> > rcu_read_locked section. As far as I know preemption needs to be
> > disabled while holding the rcu_read_lock().
> 
> Nah, a long long time ago some -rt people complained to paulmck that
> keeping preemption disabled over all this RCU stuff was killing
> latencies. Paul liked the challenge and came up with some mind twisting
> stuff to make it work.

What can I say?  I was young and foolish.  And I still am pretty
foolish.  ;-)

But yes, you are not required to disable preemption across
rcu_read_lock(), and rcu_read_lock() is not guaranteed to disable
preemption.  So if you need preemption to be disabled, do it explicitly
with preempt_disable(), local_irq_save(), rcu_read_lock_sched(),
or whatever, because rcu_read_lock() isn't always going to disable
preemption.

> If you're into that kind of pain, look at CONFIG_*_PREEMPT_RCU :-)

Or just set CONFIG_PREEMPT=y, which will set CONFIG_TREE_PREEMPT_RCU=y
on CONFIG_SMP=y builds and will set CONFIG_TINY_PREEMPT_RCU=y otherwise.
(But please note that CONFIG_TINY_PREEMPT_RCU is going away, after which
CONFIG_PREEMPT=y will always set CONFIG_TREE_PREEMPT_RCU=y.)

							Thanx, Paul

> But yeah, you need to have that stuff enabled before you can hit this
> particular snag.
> 

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