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Message-ID: <Z5PC9Rs1C2NIBR8k@localhost.localdomain>
Date: Fri, 24 Jan 2025 17:42:29 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
rostedt@...dmis.org
Subject: Re: [PATCH RFC v2 rcu] Fix get_state_synchronize_rcu_full() GP-start
detection
Le Fri, Jan 24, 2025 at 07:58:20AM -0800, Paul E. McKenney a écrit :
> On Fri, Jan 24, 2025 at 03:49:24PM +0100, Frederic Weisbecker wrote:
> > Le Fri, Dec 13, 2024 at 11:49:49AM -0800, Paul E. McKenney a écrit :
> > > The get_state_synchronize_rcu_full() and poll_state_synchronize_rcu_full()
> > > functions use the root rcu_node structure's ->gp_seq field to detect
> > > the beginnings and ends of grace periods, respectively. This choice is
> > > necessary for the poll_state_synchronize_rcu_full() function because
> > > (give or take counter wrap), the following sequence is guaranteed not
> > > to trigger:
> > >
> > > get_state_synchronize_rcu_full(&rgos);
> > > synchronize_rcu();
> > > WARN_ON_ONCE(!poll_state_synchronize_rcu_full(&rgos));
> > >
> > > The RCU callbacks that awaken synchronize_rcu() instances are
> > > guaranteed not to be invoked before the root rcu_node structure's
> > > ->gp_seq field is updated to indicate the end of the grace period.
> > > However, these callbacks might start being invoked immediately
> > > thereafter, in particular, before rcu_state.gp_seq has been updated.
> > > Therefore, poll_state_synchronize_rcu_full() must refer to the
> > > root rcu_node structure's ->gp_seq field. Because this field is
> > > updated under this structure's ->lock, any code following a call to
> > > poll_state_synchronize_rcu_full() will be fully ordered after the
> > > full grace-period computation, as is required by RCU's memory-ordering
> > > semantics.
> > >
> > > By symmetry, the get_state_synchronize_rcu_full() function should also
> > > use this same root rcu_node structure's ->gp_seq field. But it turns out
> > > that symmetry is profoundly (though extremely infrequently) destructive
> > > in this case. To see this, consider the following sequence of events:
> > >
> > > 1. CPU 0 starts a new grace period, and updates rcu_state.gp_seq
> > > accordingly.
> > >
> > > 2. As its first step of grace-period initialization, CPU 0 examines
> > > the current CPU hotplug state and decides that it need not wait
> > > for CPU 1, which is currently offline.
> > >
> > > 3. CPU 1 comes online, and updates its state. But this does not
> > > affect the current grace period, but rather the one after that.
> > > After all, CPU 1 was offline when the current grace period
> > > started, so all pre-existing RCU readers on CPU 1 must have
> > > completed or been preempted before it last went offline.
> > > The current grace period therefore has nothing it needs to wait
> > > for on CPU 1.
> > >
> > > 4. CPU 1 switches to an rcutorture kthread which is running
> > > rcutorture's rcu_torture_reader() function, which starts a new
> > > RCU reader.
> > >
> > > 5. CPU 2 is running rcutorture's rcu_torture_writer() function
> > > and collects a new polled grace-period "cookie" using
> > > get_state_synchronize_rcu_full(). Because the newly started
> > > grace period has not completed initialization, the root rcu_node
> > > structure's ->gp_seq field has not yet been updated to indicate
> > > that this new grace period has already started.
> > >
> > > This cookie is therefore set up for the end of the current grace
> > > period (rather than the end of the following grace period).
> > >
> > > 6. CPU 0 finishes grace-period initialization.
> > >
> > > 7. If CPU 1’s rcutorture reader is preempted, it will be added to
> > > the ->blkd_tasks list, but because CPU 1’s ->qsmask bit is not
> > > set in CPU 1's leaf rcu_node structure, the ->gp_tasks pointer
> > > will not be updated. Thus, this grace period will not wait on
> > > it. Which is only fair, given that the CPU did not come online
> > > until after the grace period officially started.
> > >
> > > 8. CPUs 0 and 2 then detect the new grace period and then report
> > > a quiescent state to the RCU core.
> > >
> > > 9. Because CPU 1 was offline at the start of the current grace
> > > period, CPUs 0 and 2 are the only CPUs that this grace period
> > > needs to wait on. So the grace period ends and post-grace-period
> > > cleanup starts. In particular, the root rcu_node structure's
> > > ->gp_seq field is updated to indicate that this grace period
> > > has now ended.
> > >
> > > 10. CPU 2 continues running rcu_torture_writer() and sees that,
> > > from the viewpoint of the root rcu_node structure consulted by
> > > the poll_state_synchronize_rcu_full() function, the grace period
> > > has ended. It therefore updates state accordingly.
> > >
> > > 11. CPU 1 is still running the same RCU reader, which notices this
> > > update and thus complains about the too-short grace period.
> >
> > I think I get the race but I must confess I'm not very familiar with how this
> > all materialize on CPU 2's rcu_torture_writer() and CPU 1's rcu_torture_reader().
> >
> > Basically this could trigger on CPU 1 with just doing the following, right?
> >
> > rcu_read_lock()
> > get_state_synchronize_rcu_full(&rgos);
> > WARN_ON_ONCE(poll_state_synchronize_rcu_full(&rgos))
> > rcu_read_unlock()
>
> CPU 1 would do rcu_read_lock()/checks/rcu_read_unlock() as the
> reader, and CPU 2 would do get_state_synchronize_rcu_full(), later
> poll_state_synchronize_rcu_full(), which would (erroneously) indicate
> a completed grace period, so it would update the state, triggering CPU
> 1's checks.
I see, so if I generalize the problem beyond rcutorture, this looks like this,
right?
CPU 0 CPU 1 CPU 2
----- ------ -----
rcu_seq_start(rcu_state.gp_seq)
// CPU boots
rcu_read_lock()
r0 = rcu_dereference(*X)
r1 = *X
*X = NULL
// relies on rnp->gp_seq and not rcu_state.gp_seq
get_state_synchronize_rcu_full(&gros)
WRITE_ONCE(rnp->gp_seq, rcu_state.gp_seq);
rcu_report_qs_rdp()
rcu_report_qs_rdp()
rcu_report_qs_rnp()
rcu_report_qs_rsp()
if (poll_state_synchronize_rcu_full(&rgos))
kfree(r1)
// play with r0 and crash
> > I'm wondering, what prevents us from removing rcu_state.gp_seq and rely only on
> > the root node for the global state ?
>
> One scenario comes to mind immediately. There may be others.
>
> Suppose we were running with default configuration on a system with
> "only" eight CPUs. Then there is only the one rcu_node structure,
> which is both root and leaf. Without rcu_state.gp_seq, there
> would be no way to communicate the beginning of the grace period to
> get_state_synchronize_rcu_full() without also allowing quiescent states
> to be reported. There would thus be no time in which to check for newly
> onlined/offlined CPUs.
Heh, that makes sense! Though perhaps that qsmaskinit[next] handling part
could be done before rcu_seq_start()?
Thanks.
>
> Thanx, Paul
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