From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
mingo@kernel.org, linux-kernel@vger.kernel.org, tj@kernel.org,
john.stultz@linaro.org, dimitrysh@google.com, romlem@google.com,
ccross@google.com, tkjos@google.com
Subject: Re: [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter()
Date: Thu, 21 Jul 2016 19:34:13 +0200 [thread overview]
Message-ID: <20160721173412.GA22488@redhat.com> (raw)
In-Reply-To: <20160720205851.GL7094@linux.vnet.ibm.com>
On 07/20, Paul E. McKenney wrote:
>
> On Wed, Jul 20, 2016 at 05:13:58PM +0200, Oleg Nesterov wrote:
>
> > rcu_sync_enter() or __rcu_sync_enter() is legal in any state, the latter
> > won't block.
>
> Actually, I had no idea that __rcu_sync_enter() was intended for anything
> other than internal use.
>
> Other than that, agreed, with the exception that it is illegal after
> rcu_sync_dtor() has been called.
Yes, sure. rcu_sync_dtor() "destroys" struct rcu_sync, and currently it
is only called by destroy_super_work() right before kfree(). Nothing is
legal after rcu_sync_dtor().
> > > > static void rcu_sync_call(struct rcu_sync *rsp)
> > > > {
> > > > // TODO: THIS IS SUBOPTIMAL. We want to call it directly
> > > > // if rcu_blocking_is_gp() == T, but it has might_sleep().
> > >
> > > Careful! This optimization works only for RCU-sched and RCU-bh.
> > > With normal RCU, you can be tripped up by tasks preempted within RCU
> > > read-side critical sections should CONFIG_PREEMPT=y.
> >
> > Yes, thanks, I understand ;) another reason why I do not want to add
> > this optimization into the initial version.
>
> So I should take this as a request to export rcu_blocking_is_gp()?
Would be nice ;) Or we can do something else. Nevermind, this needs
another discussion.
> > > > void rcu_sync_dtor(struct rcu_sync *rsp)
> > > > {
> > > > int gp_state;
> > > >
> > > > BUG_ON(rsp->gp_count);
> > > > BUG_ON(rsp->gp_state == GP_PASSED);
> > > >
> > > > spin_lock_irq(&rsp->rss_lock);
> > > > if (rsp->gp_state == GP_REPLAY)
> > > > rsp->gp_state = GP_EXIT;
> > >
> > > OK, this ensures that the .wait() below will wait for the callback, but
> > > it might result in some RCU read-side critical sections still being in
> > > flight after rcu_sync_dtor() completes.
> >
> > Hmm. Obviously, the caller should prevent this somehow or it is simply
> > buggy. Or I misunderstood.
>
> Hard to say without knowing what the permitted use cases are...
>
> Me, I would make rcu_sync_dtor() wait the extra grace period in this case.
> It should be a low-probability race, and it reduces the _dtor-time
> state space.
>
> What it looks like you are saying is that the caller must not only ensure
> that there will never again be a __rcu_sync_enter(), rcu_sync_enter(),
> or rcu_sync_exit() (or, I suppose, rcu_sync_dtor()) for this rcu_sync
> structure,
Yes, and
> but must also ensure that any relevant RCU read-side critical
> sections have completed.
Ah, now I understand your concerns. Yes, yes, sure. The caller must
ensure that all RCU read-side critical sections which might look at this
rcu_sync via rcu_sync_is_idle() have completed.
Currently the only caller of dtor() is percpu_free_rwsem(). So if you do,
say,
struct percpu_rw_semaphore *sem = kmalloc(...);
...
percpu_free_rwsem(sem);
kfree(sem);
you obviously need to ensure that percpu_free_rwsem() can't be called
before all readers fully complete their percpu_down_read/percpu_up_read
critical sections, this includes the RCU read-side critical sections.
And this doesn't doesn't really differ from the plain rw_semaphore.
And can't resist... let me add another "TODO" note ;) we actually want
to improve it a bit (probably just a "bool wait" arg) and kill the ugly
super_block->destroy_work which currently does percpu_free_rwsem(). This
should be simple.
Oleg.
next prev parent reply other threads:[~2016-07-21 17:34 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-14 18:25 [PATCH 0/2] locking/percpu-rwsem: Optimizations/tweaks Peter Zijlstra
2016-07-14 18:25 ` [PATCH 1/2] locking/percpu-rwsem: Optimize readers and reduce global impact Peter Zijlstra
2016-07-15 16:30 ` Oleg Nesterov
2016-07-15 19:47 ` Peter Zijlstra
2016-07-18 18:23 ` kbuild test robot
2016-07-18 22:51 ` kbuild test robot
2016-07-14 18:25 ` [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob Peter Zijlstra
2016-07-14 18:37 ` Peter Zijlstra
2016-07-14 18:43 ` Oleg Nesterov
2016-07-14 18:56 ` Peter Zijlstra
2016-07-14 19:20 ` Peter Zijlstra
2016-07-14 19:29 ` Paul E. McKenney
2016-07-14 19:38 ` Peter Zijlstra
2016-07-14 19:54 ` Paul E. McKenney
2016-07-15 13:27 ` Oleg Nesterov
2016-07-15 13:39 ` Paul E. McKenney
2016-07-15 13:45 ` Oleg Nesterov
2016-07-15 15:38 ` Paul E. McKenney
2016-07-15 16:49 ` Oleg Nesterov
2016-07-15 18:01 ` Paul E. McKenney
2016-07-16 17:10 ` [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter() Oleg Nesterov
2016-07-16 18:40 ` Oleg Nesterov
2016-07-18 11:54 ` Peter Zijlstra
2016-07-18 13:44 ` Oleg Nesterov
2016-07-19 20:50 ` Paul E. McKenney
2016-07-20 15:13 ` Oleg Nesterov
2016-07-20 20:58 ` Paul E. McKenney
2016-07-21 17:34 ` Oleg Nesterov [this message]
2016-07-20 17:16 ` Oleg Nesterov
2016-07-20 21:31 ` Paul E. McKenney
2016-07-21 17:34 ` Oleg Nesterov
2016-07-22 3:26 ` Paul E. McKenney
2016-07-25 17:01 ` Oleg Nesterov
2016-07-25 17:05 ` John Stultz
2016-07-25 17:26 ` Oleg Nesterov
2016-08-09 8:48 ` Peter Zijlstra
2016-07-25 17:49 ` Paul E. McKenney
2016-07-15 13:42 ` [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob Oleg Nesterov
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