linux-kernel - Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

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Message-ID: <20221221115924.GA34934@lothringen>
Date:   Wed, 21 Dec 2022 12:59:24 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

On Tue, Dec 20, 2022 at 10:34:19PM -0500, Mathieu Desnoyers wrote:
> On 2022-12-20 17:57, Frederic Weisbecker wrote:
> > On Tue, Dec 20, 2022 at 02:01:30PM -0500, Mathieu Desnoyers wrote:
> > > On 2022-12-20 13:29, Joel Fernandes wrote:
> > > > 
> > > 
> > > > I do want to finish my memory barrier studies of SRCU over the holidays since I have been deep in the hole with that already. Back to the post flip memory barrier here since I think now even that might not be needed…
> > > 
> > > I strongly suspect the memory barrier after flip is useless for the same
> > > reasons I mentioned explaining why the barrier before the flip is useless.
> > > 
> > > However, we need to double-check that we have memory barriers at the
> > > beginning and end of synchronize_srcu, and between load of "unlock" counters
> > > and load of "lock" counters.
> > > 
> > > Where is the barrier at the beginning of synchronize_srcu ?
> > 
> > rcu_seq_snap() ?
> 
> The memory barrier in rcu_seq_snap is not at the very beginning of synchronize_srcu.
> 
> For example we have:
> 
> unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> {
>         // Any prior manipulation of SRCU-protected data must happen
>         // before the load from ->srcu_gp_seq.
>         smp_mb();
>         return rcu_seq_snap(&ssp->srcu_gp_seq);
> 
> which happens to have an explicit barrier before issuing rcu_seq_snap().

SRCU (or even RCU) polling is special in that it may rely on a concurrent updater
to start the grace period, hence why you need two more barriers here (the second
is in poll_state_synchronize_srcu()) so that:

* If the polling updater started polling (calling get_state_synchronize_srcu())
  before the traditional updater started the grace period, the latter must
  propagate the changes from the polling updater to all CPUs.

* If the polling updater started polling (calling get_state_synchronize_srcu())
  after the traditional updater started the grace period, it must wait for a
  subsequent grace period (rcu_seq_snap() will return that subsequent sequence).

* If the polling updater checks (and thereby finishes) polling (calling poll_state_synchronize_srcu())
  after the traditional updater completes the grace period, the polling updater sees
  the propagated barrier.

* If the polling updater checks polling (calling poll_state_synchronize_srcu())
  before the traditional updater completes the grace period, keep polling.

> So my question still stands: where is the memory barrier at the beginning of
> synchronize_srcu ?

I still think rcu_seq_snap() is what you're looking for.

> 
> The memory ordering constraint I am concerned about here is:
> 
>  * [...] In addition,
>  * each CPU having an SRCU read-side critical section that extends beyond
>  * the return from synchronize_srcu() is guaranteed to have executed a
>  * full memory barrier after the beginning of synchronize_srcu() and before
>  * the beginning of that SRCU read-side critical section. [...]
> 
> So if we have a SRCU read-side critical section that begins after the beginning
> of synchronize_srcu, but before its first memory barrier, it would miss the
> guarantee that the full memory barrier is issued before the beginning of that
> SRCU read-side critical section. IOW, that memory barrier needs to be at the
> very beginning of the grace period.

I'm confused, what's wrong with this ?

UPDATER                  READER
-------                  ------
STORE X = 1              STORE srcu_read_lock++
// rcu_seq_snap()        smp_mb()
smp_mb()                 READ X
// scans
READ srcu_read_lock