From: Frederic Weisbecker <frederic@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Marco Elver <elver@google.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
rcu <rcu@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
"Uladzislau Rezki (Sony)" <urezki@gmail.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
Neeraj upadhyay <neeraj.iitr10@gmail.com>,
Alan Stern <stern@rowland.harvard.edu>
Subject: Re: [PATCH v7 6/6] rcu/segcblist: Add additional comments to explain smp_mb()
Date: Wed, 21 Oct 2020 23:16:43 +0200 [thread overview]
Message-ID: <20201021211643.GA78735@lothringen> (raw)
In-Reply-To: <CAEXW_YTtYspPNw_eL1vmGXhY8nJ8uQonSc5KuA1weYv3G+bWPg@mail.gmail.com>
On Wed, Oct 21, 2020 at 11:57:04AM -0700, Joel Fernandes wrote:
> On Mon, Oct 19, 2020 at 5:37 AM Frederic Weisbecker <frederic@kernel.org> wrote:
> > Now, reading the documentation of rcu_barrier() (thanks to you!):
> >
> > Pseudo-code using rcu_barrier() is as follows:
> >
> > 1. Prevent any new RCU callbacks from being posted.
> > 2. Execute rcu_barrier().
> > 3. Allow the module to be unloaded.
> >
>
> Basically, you are saying that if all CPUs agree that len == 0
> henceforth (through other memory barriers), then callback enqueuing
> does not need a memory barrier before setting length to 0.
I think setting length to 0 isn't much an issue. At worst we send a useless
IPI and queue a needless callback. But incrementing from 0 to 1 is precisely
what we don't want to miss.
> I think that makes sense but is it worth removing the memory barrier
> before WRITE(len, 1) and hoping after #1, the caller would have
> ensured things are fine? Also I am not sure if the above is the only
> usecase for rcu_barrier().
I'm not sure either. Also I need to check your scenario again.
> > cancel_work_sync() also seem to really sync as well. I'm less sure about del_timer_sync().
> >
> > Say we have:
> >
> > expire_timers (CPU 0) CPU 1
> > ------------- -----------
> > detach_timer(timer)
> > raw_spin_unlock(&base->lock);
> > call_timer_fn(timer, fn, baseclk);
> > -> enqueue callback
> > //would need at least smp_wmb() here
Aah, my bad, the smp_mb() after inc_len does that.
> > base->running_timer = NULL;
> >
> > del_timer_sync() {
> > raw_spin_lock(&base->lock);
> > if (base->running_timer != timer)
> > ret = detach_if_pending(timer, base, true);
> > if (!timer_pending())
> > return 0;
> > raw_spin_unlock(&base->lock);
> > }
> > //would need at least smp_rmb() here
And rcu_seq_start() implies that, although I'm not sure that's what was intended.
So we are good.
>
> Regarding "would need at least smp_rmb.." :
> But the rcu_barrier() has the control dependency we discussed in last
> emails, between READ(len) and whatever follows the rcu_barrier().
> That itself will provide the ordering right?
I'm not sure that was enough. The len itself has to be synchronized against
whatever callback enqueuer that got stopped.
> I could be missing something too :-/. But I'll include this patch in
> my next posting anyway and let us also maybe see if Paul disagrees.
Ok.
Thanks!
next prev parent reply other threads:[~2020-10-21 21:16 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-15 0:22 [PATCH v7 0/6] Add support for length of each segment in the segcblist Joel Fernandes (Google)
2020-10-15 0:22 ` [PATCH v7 1/6] rcu/tree: Make rcu_do_batch count how many callbacks were executed Joel Fernandes (Google)
2020-10-15 0:22 ` [PATCH v7 2/6] rcu/segcblist: Add counters to segcblist datastructure Joel Fernandes (Google)
2020-10-15 12:21 ` Frederic Weisbecker
2020-10-17 1:31 ` joel
2020-10-21 15:33 ` joel
2020-10-21 21:53 ` Frederic Weisbecker
2020-10-21 22:31 ` Joel Fernandes
2020-10-18 8:23 ` [rcu/segcblist] e08055898f: WARNING:at_kernel/rcu/srcutree.c:#cleanup_srcu_struct kernel test robot
2020-10-21 14:40 ` joel
2020-10-15 0:22 ` [PATCH v7 3/6] rcu/trace: Add tracing for how segcb list changes Joel Fernandes (Google)
2020-10-15 0:22 ` [PATCH v7 4/6] rcu/segcblist: Remove useless rcupdate.h include Joel Fernandes (Google)
2020-10-15 0:23 ` [PATCH v7 5/6] rcu/tree: Remove redundant smp_mb() in rcu_do_batch Joel Fernandes (Google)
2020-10-15 0:23 ` [PATCH v7 6/6] rcu/segcblist: Add additional comments to explain smp_mb() Joel Fernandes (Google)
2020-10-15 13:35 ` Frederic Weisbecker
2020-10-17 1:27 ` joel
2020-10-17 3:19 ` joel
2020-10-17 13:29 ` Frederic Weisbecker
2020-10-18 0:35 ` joel
2020-10-19 12:37 ` Frederic Weisbecker
2020-10-21 18:57 ` Joel Fernandes
2020-10-21 21:16 ` Frederic Weisbecker [this message]
2020-10-17 20:24 ` Alan Stern
2020-10-18 20:45 ` Joel Fernandes
2020-10-18 21:15 ` Alan Stern
2020-10-17 14:31 ` Alan Stern
2020-10-18 20:16 ` Joel Fernandes
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