Message-ID: <CAEXW_YSo1NTmx8FE3WNN7zL9so24o40HXhxubHJVvfxWBYtz0w@mail.gmail.com>
Date: Thu, 23 Jan 2025 20:49:47 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: paulmck@...nel.org
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com, 
	rostedt@...dmis.org
Subject: Re: [PATCH RFC rcu] Fix get_state_synchronize_rcu_full() GP-start detection

On Thu, Dec 12, 2024 at 7:59 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> 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.

This can also happen if CPU 1 is idle, and then becomes non-idle and
starts a reader which does step #3?

Reason I was asking is because it sounded from the code comment in
patch below that this issue could happen only via hotplug.

But I agree fully with the change that get.._full() should sample the
rcu_state counter and not the root node one since it more accurately
spans a full GP.

I will look through this patch again tomorrow though...
Thanks!

  - Joel



>
> 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 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.
>
> 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, 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.
>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 980f1fa719665..71620a8a2eb3d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4204,14 +4204,17 @@ 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);
>

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