Messages in this thread | | | Date | Wed, 27 Mar 2013 07:37:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: BUG: using smp_processor_id() in preemptible [00000000] code: asm/8267 |
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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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