On Thu, 2 May 2013, Paul E. McKenney wrote:
> > Only the new cond_resched_rcu() macro provides
> > PREEMPT_ACTIVE flag to skip the rcu_preempt_sleep_check()
> > call. The old macros provide locked=0 as you noticed. Does it
> > answer your question or I'm missing something?
> PREEMPT_ACTIVE's value is usually 0x10000000. Did it change
> since 3.9? If not, rcu_sleep_check(preempt_offset & PREEMPT_ACTIVE)
> is the same as rcu_sleep_check(0).
Yes, the different platforms use different bit,
that is why I mentioned about my failed attempt at
changing hardirq.h. PREEMPT_ACTIVE is always != 0.
But I don't understand what do you mean by
'preempt_offset & PREEMPT_ACTIVE' being always 0.
It is always 0 for cond_resched(), cond_resched_lock()
and cond_resched_softirq(), not for cond_resched_rcu():
(PREEMPT_ACTIVE | PREEMPT_RCU_OFFSET) & PREEMPT_ACTIVE
should give PREEMPT_ACTIVE, not 0. We have 2 cases in
rcu_sleep_check() for the if:
1. !(PREEMPT_ACTIVE) => FALSE for cond_resched_rcu
2. !(0) => TRUE for other cond_resched_* macros
On x86 the code is:
pushl %ebp #
testl $268435456, %ecx #, preempt_offset
jne .L240 #,
// rcu_lock_map checked when PREEMPT_ACTIVE is missing
// rcu_bh_lock_map checked
Julian Anastasov <ja@xxxxxx>
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