On Sun, Mar 02, 2025 at 10:46:29AM -0800, Boqun Feng wrote:
> On Sun, Mar 02, 2025 at 09:39:44AM -0800, Paul E. McKenney wrote:
> > On Sun, Mar 02, 2025 at 11:19:44AM +0100, Uladzislau Rezki wrote:
> > > On Fri, Feb 28, 2025 at 05:08:49PM -0800, Paul E. McKenney wrote:
> > > > On Fri, Feb 28, 2025 at 11:59:55AM -0800, Paul E. McKenney wrote:
> > > > > On Fri, Feb 28, 2025 at 08:12:51PM +0100, Uladzislau Rezki wrote:
> > > > > > Hello, Paul!
> > > > > >
> > > > > > > > > > >
> > > > > > > > > > > Except that I got this from overnight testing of rcu/dev
> > > > > > > > > > > on the shared
> > > > > > > > > > > RCU tree:
> > > > > > > > > > >
> > > > > > > > > > > WARNING: CPU: 5 PID: 14 at kernel/rcu/tree.c:1636
> > > > > > > > > > > rcu_sr_normal_complete+0x5c/0x80
> > > > > > > > > > >
> > > > > > > > > > > I see this only on TREE05. Which should not be too
> > > > > > > > > > > surprising, given
> > > > > > > > > > > that this is the scenario that tests it. It happened
> > > > > > > > > > > within five minutes
> > > > > > > > > > > on all 14 of the TREE05 runs.
> > > > > > > > > > >
> > > > > > > > > > Hm.. This is not fun. I tested this on my system and i did
> > > > > > > > > > not manage to
> > > > > > > > > > trigger this whereas you do. Something is wrong.
> > > > > > > > >
> > > > > > > > > If you have a debug patch, I would be happy to give it a go.
> > > > > > > > >
> > > > > > > > I can trigger it. But.
> > > > > > > >
> > > > > > > > Some background. I tested those patches during many hours on
> > > > > > > > the stable
> > > > > > > > kernel which is 6.13. On that kernel i was not able to trigger
> > > > > > > > it. Running
> > > > > > > > the rcutorture on the our shared "dev" tree, which i did now,
> > > > > > > > triggers this
> > > > > > > > right away.
> > > > > > >
> > > > > > > Bisection? (Hey, you knew that was coming!)
> > > > > > >
> > > > > > Looks like this: rcu: Fix get_state_synchronize_rcu_full() GP-start
> > > > > > detection
> > > > > >
> > > > > > After revert in the dev, rcutorture passes TREE05, 16 instances.
> > > > >
> > > > > Huh. We sure don't get to revert that one...
> > > > >
> > > > > Do we have a problem with the ordering in rcu_gp_init() between the
> > > > > calls
> > > > > to rcu_seq_start() and portions of rcu_sr_normal_gp_init()? For
> > > > > example,
> > > > > do we need to capture the relevant portion of the list before the call
> > > > > to rcu_seq_start(), and do the grace-period-start work afterwards?
> > > >
> > > > I tried moving the call to rcu_sr_normal_gp_init() before the call to
> > > > rcu_seq_start() and got no failures in a one-hour run of 200*TREE05.
> > > > Which does not necessarily mean that this is the correct fix, but I
> > > > figured that it might at least provide food for thought.
> > > >
> > > > Thanx, Paul
> > > >
> > > > ------------------------------------------------------------------------
> > > >
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 48384fa2eaeb8..d3efeff7740e7 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -1819,10 +1819,10 @@ static noinline_for_stack bool rcu_gp_init(void)
> > > >
> > > > /* Advance to a new grace period and initialize state. */
> > > > record_gp_stall_check_time();
> > > > + start_new_poll = rcu_sr_normal_gp_init();
> > > > /* Record GP times before starting GP, hence rcu_seq_start(). */
> > > > rcu_seq_start(&rcu_state.gp_seq);
> > > > ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
> > > > - start_new_poll = rcu_sr_normal_gp_init();
> > > > trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
> > > > TPS("start"));
> > > > rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
> > > > raw_spin_unlock_irq_rcu_node(rnp);
> > > >
> > > Running this 24 hours already. TREE05 * 16 scenario. I do not see any
> > > warnings yet. There is a race, indeed. The gp_seq is moved forward,
> > > wheres clients can still come until rcu_sr_normal_gp_init() places a
> > > dummy-wait-head for this GP.
> > >
> > > Thank you for testing Paul and looking to this :)
> >
> > Very good! This is a bug in this commit of mine:
> >
> > 012f47f0f806 ("rcu: Fix get_state_synchronize_rcu_full() GP-start
> > detection")
> >
> > Boqun, could you please fold this into that commit with something like
> > this added to the commit log just before the paragraph starting with
> > "Although this fixes 91a967fd6934"?
> >
> > However, simply changing get_state_synchronize_rcu_full() function
> > to use rcu_state.gp_seq instead of the root rcu_node structure's
> > ->gp_seq field results in a theoretical bug in kernels booted
> > with rcutree.rcu_normal_wake_from_gp=1 due to the following
> > sequence of events:
> >
> > o The rcu_gp_init() function invokes rcu_seq_start()
> > to officially start a new grace period.
> >
> > o A new RCU reader begins, referencing X from some
> > RCU-protected list. The new grace period is not
> > obligated to wait for this reader.
> >
> > o An updater removes X, then calls synchronize_rcu(),
> > which queues a wait element.
> >
> > o The grace period ends, awakening the updater, which
> > frees X while the reader is still referencing it.
> >
> > The reason that this is theoretical is that although the
> > grace period has officially started, none of the CPUs are
> > officially aware of this, and thus will have to assume that
> > the RCU reader pre-dated the start of the grace period.
> >
> > Except for kernels built with CONFIG_PROVE_RCU=y, which use the
> > polled grace-period APIs, which can and do complain bitterly when
> > this sequence of events occurs. Not only that, there might be
> > some future RCU grace-period mechanism that pulls this sequence
> > of events from theory into practice. This commit therefore
> > also pulls the call to rcu_sr_normal_gp_init() to precede that
> > to rcu_seq_start().
> >
> > I will let you guys decide whether the call to rcu_sr_normal_gp_init()
> > needs a comment, and, if so, what that comment should say. ;-)
> >
>
> Please see the updated patch below (next and rcu/dev branches are
> updated too).
Works for me!
> For the comment, I think we can add something like
>
> /*
> * A new wait segment must be started before gp_seq advanced, so
> * that previous gp waiters won't observe the new gp_seq.
> */
>
> but I will let Ulad to decide ;-)
Over to you, Uladzislau! ;-)
Thanx, Paul
> Regards,
> Boqun
>
> ------------------------>8
> Subject: [PATCH] rcu: Fix get_state_synchronize_rcu_full() GP-start detection
>
> 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.
>
> The fix is for the get_state_synchronize_rcu_full() function to use
> rcu_state.gp_seq instead of the root rcu_node structure's ->gp_seq field.
> With this change in place, if step 5's cookie indicates that the grace
> period has not yet started, then any prior code executed by CPU 2 must
> have happened before CPU 1 came online. This will in turn prevent CPU
> 1's code in steps 3 and 11 from spanning CPU 2's grace-period wait,
> thus preventing CPU 1 from being subjected to a too-short grace period.
>
> This commit therefore makes this change. Note that there is no change to
> the poll_state_synchronize_rcu_full() function, which as noted above,
> must continue to use the root rcu_node structure's ->gp_seq field.
> This is of course an asymmetry between these two functions, but is an
> asymmetry that is absolutely required for correct operation. It is a
> common human tendency to greatly value symmetry, and sometimes symmetry
> is a wonderful thing. Other times, symmetry results in poor performance.
> But in this case, symmetry is just plain wrong.
>
> Nevertheless, the asymmetry does require an additional adjustment.
> It is possible for get_state_synchronize_rcu_full() to see a given
> grace period as having started, but for an immediately following
> poll_state_synchronize_rcu_full() to see it as having not yet started.
> Given the current rcu_seq_done_exact() implementation, this will
> result in a false-positive indication that the grace period is done
> from poll_state_synchronize_rcu_full(). This is dealt with by making
> rcu_seq_done_exact() reach back three grace periods rather than just
> two of them.
>
> However, simply changing get_state_synchronize_rcu_full() function to
> use rcu_state.gp_seq instead of the root rcu_node structure's ->gp_seq
> field results in a theoretical bug in kernels booted with
> rcutree.rcu_normal_wake_from_gp=1 due to the following sequence of
> events:
>
> o The rcu_gp_init() function invokes rcu_seq_start() to officially
> start a new grace period.
>
> o A new RCU reader begins, referencing X from some RCU-protected
> list. The new grace period is not obligated to wait for this
> reader.
>
> o An updater removes X, then calls synchronize_rcu(), which queues
> a wait element.
>
> o The grace period ends, awakening the updater, which frees X
> while the reader is still referencing it.
>
> The reason that this is theoretical is that although the grace period
> has officially started, none of the CPUs are officially aware of this,
> and thus will have to assume that the RCU reader pre-dated the start of
> the grace period.
>
> Except for kernels built with CONFIG_PROVE_RCU=y, which use the polled
> grace-period APIs, which can and do complain bitterly when this sequence
> of events occurs. Not only that, there might be some future RCU
> grace-period mechanism that pulls this sequence of events from theory
> into practice. This commit therefore also pulls the call to
> rcu_sr_normal_gp_init() to precede that to rcu_seq_start().
>
> Although this fixes 91a967fd6934 ("rcu: Add full-sized polling for
> get_completed*() and poll_state*()"), it is not clear that it is
> worth backporting this commit. First, it took me many weeks to
> convince rcutorture to reproduce this more frequently than once
> per year. Second, this cannot be reproduced at all without frequent
> CPU-hotplug operations, as in waiting all of 50 milliseconds from the
> end of the previous operation until starting the next one. Third,
> the TREE03.boot settings cause multi-millisecond delays during RCU
> grace-period initialization, which greatly increase the probability of
> the above sequence of events. (Don't do this in production workloads!)
> Fourth, the TREE03 rcutorture scenario was modified to use four-CPU
> guest OSes, to have a single-rcu_node combining tree, no testing of RCU
> priority boosting, and no random preemption, and these modifications
> were necessary to reproduce this issue in a reasonable timeframe.
> Fifth, extremely heavy use of get_state_synchronize_rcu_full() and/or
> poll_state_synchronize_rcu_full() is required to reproduce this, and as of
> v6.12, only kfree_rcu() uses it, and even then not particularly heavily.
>
> [boqun: Apply the fix [1]]
>
> Signed-off-by: Paul E. McKenney <paul...@kernel.org>
> Reviewed-by: Frederic Weisbecker <frede...@kernel.org>
> Reviewed-by: Joel Fernandes (Google) <j...@joelfernandes.org>
> Tested-by: Uladzislau Rezki (Sony) <ure...@gmail.com>
> Link:
> https://lore.kernel.org/rcu/d90bd6d9-d15c-4b9b-8a69-95336e74e8f4@paulmck-laptop/
> [1]
> Signed-off-by: Boqun Feng <boqun.f...@gmail.com>
> ---
> kernel/rcu/rcu.h | 2 +-
> kernel/rcu/tree.c | 11 +++++++----
> 2 files changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index feb3ac1dc5d5..f87c9d6d36fc 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -162,7 +162,7 @@ static inline bool rcu_seq_done_exact(unsigned long *sp,
> unsigned long s)
> {
> unsigned long cur_s = READ_ONCE(*sp);
>
> - return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (2 *
> RCU_SEQ_STATE_MASK + 1));
> + return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (3 *
> RCU_SEQ_STATE_MASK + 1));
> }
>
> /*
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index e4c0ce600b2b..f80cfdc3ee3e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1801,10 +1801,10 @@ static noinline_for_stack bool rcu_gp_init(void)
>
> /* Advance to a new grace period and initialize state. */
> record_gp_stall_check_time();
> + start_new_poll = rcu_sr_normal_gp_init();
> /* Record GP times before starting GP, hence rcu_seq_start(). */
> rcu_seq_start(&rcu_state.gp_seq);
> ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
> - start_new_poll = rcu_sr_normal_gp_init();
> trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
> rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
> raw_spin_unlock_irq_rcu_node(rnp);
> @@ -3357,14 +3357,17 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
> */
> void get_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
> {
> - struct rcu_node *rnp = rcu_get_root();
> -
> /*
> * Any prior manipulation of RCU-protected data must happen
> * before the loads from ->gp_seq and ->expedited_sequence.
> */
> smp_mb(); /* ^^^ */
> - rgosp->rgos_norm = rcu_seq_snap(&rnp->gp_seq);
> +
> + // Yes, rcu_state.gp_seq, not rnp_root->gp_seq, the latter's use
> + // in poll_state_synchronize_rcu_full() notwithstanding. Use of
> + // the latter here would result in too-short grace periods due to
> + // interactions with newly onlined CPUs.
> + rgosp->rgos_norm = rcu_seq_snap(&rcu_state.gp_seq);
> rgosp->rgos_exp = rcu_seq_snap(&rcu_state.expedited_sequence);
> }
> EXPORT_SYMBOL_GPL(get_state_synchronize_rcu_full);
> --
> 2.45.1
>
