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

From: Neeraj Upadhyay
Date: Thu Dec 09 2021 - 13:31:56 EST

Hi,

On 12/9/2021 8:39 PM, 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.

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().

Can we drop this smp_mb(),as arch_spin_lock(&rcu_state.ofl_lock) provides the ordering now?

- 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);

'flags' is uninitialized now?



Thanks
Neeraj

}
- 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. */
};