linux-kernel - Re: [PATCH] rcu/doc: Add a quick quiz to explain further why we need smp_mb__after_unlock

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Message-ID: <20210611224517.GA150081@lothringen>
Date:   Sat, 12 Jun 2021 00:45:17 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Uladzislau Rezki <urezki@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH] rcu/doc: Add a quick quiz to explain further why we need
 smp_mb__after_unlock_lock()

On Fri, Jun 11, 2021 at 10:25:14AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 11, 2021 at 12:34:32PM +0200, Frederic Weisbecker wrote:
> Glad to help, and I will reach out to you should someone make the mistake
> of insisting that I write something in French.  ;-)

If that can help, we still have frenglish for neutral territories such as airports.
Not easy to master though...

> 
> > > ++-----------------------------------------------------------------------+
> > > +
> > >  This approach must be extended to include idle CPUs, which need
> > >  RCU's grace-period memory ordering guarantee to extend to any
> > >  RCU read-side critical sections preceding and following the current
> 
> How about like this?
> 
> +-----------------------------------------------------------------------+
> | **Quick Quiz**:                                                       |
> +-----------------------------------------------------------------------+
> | But the chain of rcu_node-structure lock acquisitions guarantees      |
> | that new readers will see all of the updater's pre-grace-period       |
> | accesses and also guarantees that the updater's post-grace-period     |
> | accesses will see all of the old reader's accesses.  So why do we     |
> | need all of those calls to smp_mb__after_unlock_lock()?               |
> +-----------------------------------------------------------------------+
> | **Answer**:                                                           |
> +-----------------------------------------------------------------------+
> | Because we must provide ordering for RCU's polling grace-period       |
> | primitives, for example, get_state_synchronize_rcu() and              |
> | poll_state_synchronize_rcu().  Consider this code::                   |
> |                                                                       |
> |  CPU 0                                     CPU 1                      |
> |  ----                                      ----                       |
> |  WRITE_ONCE(X, 1)                          WRITE_ONCE(Y, 1)           |
> |  g = get_state_synchronize_rcu()           smp_mb()                   |
> |  while (!poll_state_synchronize_rcu(g))    r1 = READ_ONCE(X)          |
> |          continue;                                                    |
> |  r0 = READ_ONCE(Y)                                                    |
> |                                                                       |
> | RCU guarantees that the outcome r0 == 0 && r1 == 0 will not           |
> | happen, even if CPU 1 is in an RCU extended quiescent state           |
> | (idle or offline) and thus won't interact directly with the RCU       |
> | core processing at all.                                               |
> +-----------------------------------------------------------------------+

Very good, thanks a lot :o)