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Message-ID: <CALm+0cW=1PAVhjHKM+aXLHHzsD0+7pbVDvmm+kxvff7r=Di2tA@mail.gmail.com>
Date: Mon, 21 Oct 2024 08:39:24 +0800
From: Z qiang <qiang.zhang1211@...il.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: paulmck@...nel.org, neeraj.upadhyay@...nel.org, joel@...lfernandes.org,
urezki@...il.com, boqun.feng@...il.com, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/nocb: Fix the WARN_ON_ONCE() in rcu_nocb_rdp_deoffload()
>
> Le Sun, Oct 20, 2024 at 08:51:19PM +0800, Zqiang a écrit :
> > Currently, running rcutorture test with torture_type=rcu fwd_progress=8
> > n_barrier_cbs=8 nocbs_nthreads=8 nocbs_toggle=100 onoff_interval=60
> > test_boost=2, will trigger the following warning:
> >
> > WARNING: CPU: 19 PID: 100 at kernel/rcu/tree_nocb.h:1061 rcu_nocb_rdp_deoffload+0x292/0x2a0
> > RIP: 0010:rcu_nocb_rdp_deoffload+0x292/0x2a0
> > [18839.537322] Call Trace:
> > [18839.538006] <TASK>
> > [18839.538596] ? __warn+0x7e/0x120
> > [18839.539491] ? rcu_nocb_rdp_deoffload+0x292/0x2a0
> > [18839.540757] ? report_bug+0x18e/0x1a0
> > [18839.541805] ? handle_bug+0x3d/0x70
> > [18839.542837] ? exc_invalid_op+0x18/0x70
> > [18839.543959] ? asm_exc_invalid_op+0x1a/0x20
> > [18839.545165] ? rcu_nocb_rdp_deoffload+0x292/0x2a0
> > [18839.546547] rcu_nocb_cpu_deoffload+0x70/0xa0
> > [18839.547814] rcu_nocb_toggle+0x136/0x1c0
> > [18839.548960] ? __pfx_rcu_nocb_toggle+0x10/0x10
> > [18839.550073] kthread+0xd1/0x100
> > [18839.550958] ? __pfx_kthread+0x10/0x10
> > [18839.552008] ret_from_fork+0x2f/0x50
> > [18839.553002] ? __pfx_kthread+0x10/0x10
> > [18839.553968] ret_from_fork_asm+0x1a/0x30
> > [18839.555038] </TASK>
> >
> > CPU0 CPU2 CPU3
> > //rcu_nocb_toggle //nocb_cb_wait //rcutorture
> >
> > // deoffload CPU1 // process CPU1's rdp
> > rcu_barrier()
> > rcu_segcblist_entrain()
> > rcu_segcblist_add_len(1);
> > // len == 2
> > // enqueue barrier
> > // callback to CPU1's
> > // rdp->cblist
> > rcu_do_batch()
> > // invoke CPU1's rdp->cblist
> > // callback
> > rcu_barrier_callback()
> > rcu_barrier()
> > mutex_lock(&rcu_state.barrier_mutex);
> > // still see len == 2
> > // enqueue barrier callback
> > // to CPU1's rdp->cblist
> > rcu_segcblist_entrain()
> > rcu_segcblist_add_len(1);
> > // len == 3
> > // decrement len
> > rcu_segcblist_add_len(-2);
> > kthread_parkme()
> >
> > // CPU1's rdp->cblist len == 1
> > // Warn because there is
> > // still a pending barrier
> > // trigger warning
> > WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
> > cpus_read_unlock();
> >
> > // wait CPU1 comes online
> > // invoke barrier callback on
> > // CPU1 rdp's->cblist
> > wait_for_completion(&rcu_state.barrier_completion);
> > // deoffload CPU4
> > cpus_read_lock()
> > rcu_barrier()
> > mutex_lock(&rcu_state.barrier_mutex);
> > // block on barrier_mutex
> > // wait rcu_barrier() on
> > // CPU3 to unlock barrier_mutex
> > // but CPU3 unlock barrier_mutex
> > // need to wait CPU1 comes online
> > // when CPU1 going online will block on cpus_write_lock
> >
> > The above scenario will not only trigger WARN_ON_ONCE(), but also
> > trigger deadlock, this commit therefore check rdp->cblist length
> > before invoke kthread_parkme(), and the kthread_parkme() is not
> > invoke until length reaches zero.
> >
> > Signed-off-by: Zqiang <qiang.zhang1211@...il.com>
> > ---
> > kernel/rcu/tree_nocb.h | 8 +++++++-
> > 1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 8648233e1717..a2b0ebdefee3 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -893,6 +893,12 @@ static inline bool nocb_cb_wait_cond(struct rcu_data *rdp)
> > return !READ_ONCE(rdp->nocb_cb_sleep) || kthread_should_park();
> > }
> >
> > +static inline bool nocb_cblist_empty(struct rcu_data *rdp)
> > +{
> > + return !(rcu_rdp_is_offloaded(rdp) &&
>
> But the rdp has to be offloaded when nocb_cb_wait() is running, and that
> include the times when it is parking and when it is unparking.
>
> > + WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist)));
>
> And like your scenario above shows, it's possible to reach here with
> callbacks. So this check shouldn't be a warning at that point?
Yes, the WARN_ON_ONCE() should be removed.
>
> > +}
> > +
> > /*
> > * Invoke any ready callbacks from the corresponding no-CBs CPU,
> > * then, if there are no more, wait for more to appear.
> > @@ -907,7 +913,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
> >
> > swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
> > nocb_cb_wait_cond(rdp));
> > - if (kthread_should_park()) {
> > + if (kthread_should_park() && nocb_cblist_empty(rdp)) {
>
> What about this instead? If the second barrier is queued before
> the final check to rcu_segcblist_ready_cbs() in nocb_cb_wait(), this
> will be noticed and ->nocb_cb_sleep will remain false. If otherwise rcu_barrier()
> is called after that final rcu_segcblist_ready_cbs() check, it will observe
> the final decrement to zero and won't entrain the callback.
>
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 16865475120b..0de07d44646c 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -891,7 +891,19 @@ static void nocb_cb_wait(struct rcu_data *rdp)
> swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
> nocb_cb_wait_cond(rdp));
> if (kthread_should_park()) {
> - kthread_parkme();
> + /*
> + * kthread_park() must be preceded by an rcu_barrier().
> + * But yet another rcu_barrier() might have sneaked in between
> + * the barrier callback execution and the callbacks counter
> + * decrement.
> + */
> + if (rdp->nocb_cb_sleep) {
For the non-nocb cpus set during boot, the corresponding
rcuop kthread, we should park directly, otherwise
WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp)) will be triggered.
Should the conditions be like this?
if(!rcu_rdp_is_offloaded(rdp) || rdp->nocb_cb_sleep)
Thanks
Zqiang
> + rcu_nocb_lock_irqsave(rdp, flags);
> + WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
> + rcu_nocb_unlock_irqrestore(rdp, flags);
> +
> + kthread_parkme();
> + }
> } else if (READ_ONCE(rdp->nocb_cb_sleep)) {
> WARN_ON(signal_pending(current));
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
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