Messages in this thread | | | Subject | Re: [PATCH] updated low-latency zap_page_range | From | Robert Love <> | Date | 24 Jul 2002 18:16:24 -0700 |
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On Wed, 2002-07-24 at 17:45, Andrew Morton wrote:
> Robert Love wrote: > > > > +static inline void cond_resched_lock(spinlock_t * lock) > > +{ > > + if (need_resched() && preempt_count() == 1) { > > + _raw_spin_unlock(lock); > > + preempt_enable_no_resched(); > > + __cond_resched(); > > + spin_lock(lock); > > + } > > +} > > Maybe I'm being thick. How come a simple spin_unlock() in here > won't do the right thing?
It will, but we will check need_resched twice. And preempt_count again. My original version just did the "unlock; lock" combo and thus the checking was automatic... but if we want to check before we unlock, we might as well be optimal about it.
> And this won't _really_ compile to nothing with CONFIG_PREEMPT=n, > will it? It just does nothing because preempt_count() is zero?
I hope it compiles to nothing! There is a false in an if... oh, wait, to preserve possible side-effects gcc will keep the need_resched() call so I guess we should reorder it as:
if (preempt_count() == 1 && need_resched())
Then we get "if (0 && ..)" which should hopefully be evaluated away. Then the inline is empty and nothing need be done.
Robert Love
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