Hello,
On Fri, 3 May 2013, Paul E. McKenney wrote:
> OK, after getting some sleep, I might have located the root cause of
> my confusion yesterday.
>
> The key point is that I don't understand why we cannot get the effect
> we are looking for with the following in sched.h (or wherever):
>
> static inline int cond_resched_rcu(void)
> {
> #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
> rcu_read_unlock();
> cond_resched();
> rcu_read_lock();
> #endif
> }
>
> This adds absolutely no overhead in non-debug builds of CONFIG_PREEMPT_RCU,
> does the checking in debug builds, and allows voluntary preemption in
> !CONFIG_PREEMPT_RCU builds. CONFIG_PROVE_RCU builds will check for an
> (illegal) outer rcu_read_lock() in CONFIG_PREEMPT_RCU builds, and you
> will get "scheduling while atomic" in response to an outer rcu_read_lock()
> in !CONFIG_PREEMPT_RCU builds.
>
> It also seems to me a lot simpler.
>
> Does this work, or am I still missing something?
It should work. It is a better version of
the 2nd variant I mentioned here:
http://marc.info/?l=linux-netdev&m=136741839021257&w=2
I'll stick to this version, hope Peter Zijlstra
agrees. Playing with PREEMPT_ACTIVE or another bit makes
the things more complex.
To summarize:
- CONFIG_PREEMPT_RCU:
- no empty functions called
- CONFIG_DEBUG_ATOMIC_SLEEP can catch errors even
for this case
- non-CONFIG_PREEMPT_RCU:
- rcu_read_lock and rcu_read_unlock are barrier(),
so it expands just to cond_resched()
I'll repeat the tests tomorrow and if there are
no problems will post official version after the merge window.
Regards
--
Julian Anastasov <ja@xxxxxx>
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