Messages in this thread | | | Date | Sat, 30 May 2015 21:25:31 +0200 | From | Oleg Nesterov <> | Subject | Re: [RFC][PATCH 1/5] rcu: Create rcu_sync infrastructure |
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On 05/30, Oleg Nesterov wrote: > > On 05/30, Paul E. McKenney wrote: > > > > On Tue, May 26, 2015 at 01:43:57PM +0200, Peter Zijlstra wrote: > > > From: Oleg Nesterov <oleg@redhat.com> > > > > > > It is functionally equivalent to > > > > > > struct rcu_sync_struct { > > > atomic_t counter; > > > }; > > > > > > static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss) > > > { > > > return atomic_read(&rss->counter) == 0; > > > } > > > > > > static inline void rcu_sync_enter(struct rcu_sync_struct *rss) > > > { > > > atomic_inc(&rss->counter); > > > synchronize_sched(); > > > } > > > > For vanilla RCU, this is called get_state_synchronize_rcu(). > > > > > static inline void rcu_sync_exit(struct rcu_sync_struct *rss) > > > { > > > synchronize_sched(); > > > atomic_dec(&rss->counter); > > > } > > > > > > except: it records the state and synchronize_sched() is only called by > > > rcu_sync_enter() and only if necessary. > > > > Again for vanilla RCU, this is called cond_synchronize_rcu(). > > Hmm. I do not understand... I think rcu_sync doesn't need > get_state/cond_synchronize. > > The first caller of rcu_sync_enter() always needs sync(). The next one > could use cond_synchronize_rcu(), but for what? The 2nd one will wait > for the end of gp started by the first caller, and this is more optimal? > > Note that rcu_sync_enter/rcu_sync_func never call sync() unless strictly > necessary.
If you meant that rcu_sync_exit() could use cond_synchronize_rcu(), this doesn't look right too... We always need another synchronize_rcu() after the last writer does rcu_sync_exit(). Except rcu_sync_exit() uses call_rcu() and thus it never blocks.
> Or I misunderstood you?
Yes...
Oleg.
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