Messages in this thread | | | Date | Mon, 9 Sep 2013 15:29:02 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section? |
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On Mon, Sep 09, 2013 at 03:14:52PM +0200, Peter Zijlstra wrote: > On Mon, Sep 09, 2013 at 08:55:04AM -0400, Steven Rostedt wrote: > > On Mon, 9 Sep 2013 14:45:49 +0200 > > Frederic Weisbecker <fweisbec@gmail.com> wrote: > > > > > > > > This just proves that the caller of rcu_is_cpu_idle() must disable > > > > preemption itself for the entire time that it needs to use the result > > > > of rcu_is_cpu_idle(). > > > > > > Sorry, I don't understand your point here. What's wrong with checking the > > > ret from another CPU? > > > > Hmm, OK, this is why that code is in desperate need of a comment. > > > > From reading the context a bit more, it seems that the per cpu value is > > more a "per task" value that happens to be using per cpu variables, and > > changes on context switches. Is that correct? > > > > Anyway, it requires a comment to explain that we are not checking the > > CPU state, but really the current task state, otherwise that 'ret' > > value wouldn't travel with the task, but would stick with the CPU. > > Egads.. and the only reason we couldn't do the immediate load is because > of that atomic mess. > > Also, if its per-task, why don't we have this in the task struct? The > current scheme makes the context switch more expensive -- is this the > right trade-off?
No, putting that on the task_struct won't help much in this regard I think. Regular schedule() calls don't change that per cpu state.
Only preempt_schedule_irq() and schedule_user() are concerned with rcu eqs state exit/restore. But still storing that on task struct won't help.
> > So maybe something like: > > int rcu_is_cpu_idle(void) > { > /* > * Comment explaining that rcu_dynticks.dynticks really is a > * per-task something and we need preemption-safe loading. > */ > atomic_t dynticks = this_cpu_read(rcu_dynticks.dynticks); > return !(__atomic_read(&dynticks) & 0x01); > } > > Where __atomic_read() would be like atomic_read() but without the > volatile crap since that's entirely redundant here I think. > > The this_cpu_read() should ensure we get a preemption-safe copy of the > value. > > Once that this_cpu stuff grows preemption checks we'd need something > like __raw_this_cpu_read() or whatever the variant without preemption > checks will be called.
Yeah I thought about using this_cpu_read() too, lets wait for the preemption checks to get in.
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