Re: [PATCH 02/11] rcu: Kill rnp->ofl_seq and use only rcu_state.ofl_lock for exclusion

From: Paul E. McKenney
Date: Thu Dec 09 2021 - 12:18:44 EST

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On Thu, Dec 09, 2021 at 03:09:29PM +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> If we allow architectures to bring APs online in parallel, then we end
> up requiring rcu_cpu_starting() to be reentrant. But currently, the
> manipulation of rnp->ofl_seq is not thread-safe.
>
> However, rnp->ofl_seq is also fairly much pointless anyway since both
> rcu_cpu_starting() and rcu_report_dead() hold rcu_state.ofl_lock for
> fairly much the whole time that rnp->ofl_seq is set to an odd number
> to indicate that an operation is in progress.
>
> So drop rnp->ofl_seq completely, and use only rcu_state.ofl_lock.
>
> This has a couple of minor complexities: lockdep will complain when we
> take rcu_state.ofl_lock, and currently accepts the 'excuse' of having
> an odd value in rnp->ofl_seq. So switch it to an arch_spinlock_t to
> avoid that false positive complaint. Since we're killing rnp->ofl_seq
> of course that 'excuse' has to be changed too, so make it check for
> arch_spin_is_locked(rcu_state.ofl_lock).
>
> There's no arch_spin_lock_irqsave() so we have to manually save and
> restore local interrupts around the locking.
>
> At Paul's request, make rcu_gp_init not just wait but *exclude* any
> CPU online/offline activity, which was fairly much true already by
> virtue of it holding rcu_state.ofl_lock.

Looks good!

Could you please also make the first clause read something like this?

"At Paul's request based on Neeraj's analysis, ..."

I am going to pull this into -rcu for more intensive testing of this
code for non-concurrent CPU-online operations (making the above change
to the commit log). At some point, rcutorture needs to learn how to
do concurrent CPU-online operations, but it would be good to bake the
RCU-specific patches for a bit beforehand.

Depending on timing, you might wish to send this patch with the
rest of this group, so:

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

If testing goes well and if you don't get it there first, I expect
to push this during the v5.18 merge window.

Thanx, Paul

> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
> kernel/rcu/tree.c | 64 +++++++++++++++++++++++------------------------
> kernel/rcu/tree.h | 4 +--
> 2 files changed, 32 insertions(+), 36 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index ef8d36f580fc..a1bb0b1229ed 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -91,7 +91,7 @@ static struct rcu_state rcu_state = {
> .abbr = RCU_ABBR,
> .exp_mutex = __MUTEX_INITIALIZER(rcu_state.exp_mutex),
> .exp_wake_mutex = __MUTEX_INITIALIZER(rcu_state.exp_wake_mutex),
> - .ofl_lock = __RAW_SPIN_LOCK_UNLOCKED(rcu_state.ofl_lock),
> + .ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,
> };
>
> /* Dump rcu_node combining tree at boot to verify correct setup. */
> @@ -1168,7 +1168,15 @@ bool rcu_lockdep_current_cpu_online(void)
> preempt_disable_notrace();
> rdp = this_cpu_ptr(&rcu_data);
> rnp = rdp->mynode;
> - if (rdp->grpmask & rcu_rnp_online_cpus(rnp) || READ_ONCE(rnp->ofl_seq) & 0x1)
> + /*
> + * Strictly, we care here about the case where the current CPU is
> + * in rcu_cpu_starting() and thus has an excuse for rdp->grpmask
> + * not being up to date. So arch_spin_is_locked() might have a
> + * false positive if it's held by some *other* CPU, but that's
> + * OK because that just means a false *negative* on the warning.
> + */
> + if (rdp->grpmask & rcu_rnp_online_cpus(rnp) ||
> + arch_spin_is_locked(&rcu_state.ofl_lock))
> ret = true;
> preempt_enable_notrace();
> return ret;
> @@ -1731,7 +1739,6 @@ static void rcu_strict_gp_boundary(void *unused)
> */
> static noinline_for_stack bool rcu_gp_init(void)
> {
> - unsigned long firstseq;
> unsigned long flags;
> unsigned long oldmask;
> unsigned long mask;
> @@ -1774,22 +1781,17 @@ static noinline_for_stack bool rcu_gp_init(void)
> * of RCU's Requirements documentation.
> */
> WRITE_ONCE(rcu_state.gp_state, RCU_GP_ONOFF);
> + /* Exclude CPU hotplug operations. */
> rcu_for_each_leaf_node(rnp) {
> - // Wait for CPU-hotplug operations that might have
> - // started before this grace period did.
> - smp_mb(); // Pair with barriers used when updating ->ofl_seq to odd values.
> - firstseq = READ_ONCE(rnp->ofl_seq);
> - if (firstseq & 0x1)
> - while (firstseq == READ_ONCE(rnp->ofl_seq))
> - schedule_timeout_idle(1); // Can't wake unless RCU is watching.
> - smp_mb(); // Pair with barriers used when updating ->ofl_seq to even values.
> - raw_spin_lock(&rcu_state.ofl_lock);
> - raw_spin_lock_irq_rcu_node(rnp);
> + local_irq_save(flags);
> + arch_spin_lock(&rcu_state.ofl_lock);
> + raw_spin_lock_rcu_node(rnp);
> if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
> !rnp->wait_blkd_tasks) {
> /* Nothing to do on this leaf rcu_node structure. */
> - raw_spin_unlock_irq_rcu_node(rnp);
> - raw_spin_unlock(&rcu_state.ofl_lock);
> + raw_spin_unlock_rcu_node(rnp);
> + arch_spin_unlock(&rcu_state.ofl_lock);
> + local_irq_restore(flags);
> continue;
> }
>
> @@ -1824,8 +1826,9 @@ static noinline_for_stack bool rcu_gp_init(void)
> rcu_cleanup_dead_rnp(rnp);
> }
>
> - raw_spin_unlock_irq_rcu_node(rnp);
> - raw_spin_unlock(&rcu_state.ofl_lock);
> + raw_spin_unlock_rcu_node(rnp);
> + arch_spin_unlock(&rcu_state.ofl_lock);
> + local_irq_restore(flags);
> }
> rcu_gp_slow(gp_preinit_delay); /* Races with CPU hotplug. */
>
> @@ -4233,7 +4236,7 @@ int rcutree_offline_cpu(unsigned int cpu)
> */
> void rcu_cpu_starting(unsigned int cpu)
> {
> - unsigned long flags;
> + unsigned long flags, seq_flags;
> unsigned long mask;
> struct rcu_data *rdp;
> struct rcu_node *rnp;
> @@ -4246,11 +4249,11 @@ void rcu_cpu_starting(unsigned int cpu)
>
> rnp = rdp->mynode;
> mask = rdp->grpmask;
> - WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
> - WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));
> + local_irq_save(seq_flags);
> + arch_spin_lock(&rcu_state.ofl_lock);
> rcu_dynticks_eqs_online();
> smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
> - raw_spin_lock_irqsave_rcu_node(rnp, flags);
> + raw_spin_lock_rcu_node(rnp);
> WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
> newcpu = !(rnp->expmaskinitnext & mask);
> rnp->expmaskinitnext |= mask;
> @@ -4269,9 +4272,8 @@ void rcu_cpu_starting(unsigned int cpu)
> } else {
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> }
> - smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
> - WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
> - WARN_ON_ONCE(rnp->ofl_seq & 0x1);
> + arch_spin_unlock(&rcu_state.ofl_lock);
> + local_irq_restore(seq_flags);
> smp_mb(); /* Ensure RCU read-side usage follows above initialization. */
> }
>
> @@ -4285,7 +4287,7 @@ void rcu_cpu_starting(unsigned int cpu)
> */
> void rcu_report_dead(unsigned int cpu)
> {
> - unsigned long flags;
> + unsigned long flags, seq_flags;
> unsigned long mask;
> struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */
> @@ -4299,10 +4301,8 @@ void rcu_report_dead(unsigned int cpu)
>
> /* Remove outgoing CPU from mask in the leaf rcu_node structure. */
> mask = rdp->grpmask;
> - WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
> - WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));
> - smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
> - raw_spin_lock(&rcu_state.ofl_lock);
> + local_irq_save(seq_flags);
> + arch_spin_lock(&rcu_state.ofl_lock);
> raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */
> rdp->rcu_ofl_gp_seq = READ_ONCE(rcu_state.gp_seq);
> rdp->rcu_ofl_gp_flags = READ_ONCE(rcu_state.gp_flags);
> @@ -4313,10 +4313,8 @@ void rcu_report_dead(unsigned int cpu)
> }
> WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask);
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> - raw_spin_unlock(&rcu_state.ofl_lock);
> - smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
> - WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
> - WARN_ON_ONCE(rnp->ofl_seq & 0x1);
> + arch_spin_unlock(&rcu_state.ofl_lock);
> + local_irq_restore(seq_flags);
>
> rdp->cpu_started = false;
> }
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 305cf6aeb408..aff4cc9303fb 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -56,8 +56,6 @@ struct rcu_node {
> /* Initialized from ->qsmaskinitnext at the */
> /* beginning of each grace period. */
> unsigned long qsmaskinitnext;
> - unsigned long ofl_seq; /* CPU-hotplug operation sequence count. */
> - /* Online CPUs for next grace period. */
> unsigned long expmask; /* CPUs or groups that need to check in */
> /* to allow the current expedited GP */
> /* to complete. */
> @@ -358,7 +356,7 @@ struct rcu_state {
> const char *name; /* Name of structure. */
> char abbr; /* Abbreviated name. */
>
> - raw_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
> + arch_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
> /* Synchronize offline with */
> /* GP pre-initialization. */
> };
> --
> 2.31.1
>

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Previous message: André Almeida: "Re: [PATCH v4] futex: Clarify comment at futex_wait_multiple_setup()"
In reply to: David Woodhouse: "[PATCH 02/11] rcu: Kill rnp->ofl_seq and use only rcu_state.ofl_lock for exclusion"
Next in thread: David Woodhouse: "Re: [PATCH 02/11] rcu: Kill rnp->ofl_seq and use only rcu_state.ofl_lock for exclusion"
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