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Subject[PATCH] rcu/tree: Correctly handle single cpu check in rcu_blocking_is_gp
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Currently, for non-preempt kernels (with CONFIG_PREEMPTION=n),
rcu_blocking_is_gp() checks (with preemption disabled), whether
there is only one cpu online. It uses num_online_cpus() to
decide whether only one cpu is online. If there is only single
cpu online, synchronize_rcu() is optimized to return without
doing all the work to wait for grace period. However, there are
few issues with the num_online_cpus() check used:

1. num_online_cpus() does a atomic_read(&__num_online_cpus). As
hotplug locks are not held, this does not ensure that
new incoming cpus update of the count is visible. This can
result in read side section on new incoming cpu, observe
updates which should not be visible beyond the grace period
corresponding to synchronize_rcu().

For e.g. below litmus test, where P0 process corresponds to
synchronize_rcu() and P1 corresponds to new online cpu, has
positive witnesses; confirming the possibility of read side
section to extend before and after the grace period, thereby
breaking guarantees provided by synchronize_rcu().

{
int x = 0;
atomic_t numonline = ATOMIC_INIT(1);
}

P0(int *x, atomic_t *numonline)
{
int r0;
WRITE_ONCE(*x, 1);
r0 = atomic_read(numonline);
if (r0 == 1) {
smp_mb();
} else {
synchronize_rcu();
}
WRITE_ONCE(*x, 2);
}

P1(int *x, atomic_t *numonline)
{
int r0; int r1;

atomic_inc(numonline);
smp_mb();
rcu_read_lock();
r0 = READ_ONCE(*x);
smp_rmb();
r1 = READ_ONCE(*x);
rcu_read_unlock();
}

locations [x;numonline;]

exists (1:r0=0 /\ 1:r1=2)

2. Second problem is, the same early exit, from synchronize_rcu()
does not provide full ordering, memory barrier, w.r.t. memory
accesses after synchronize_rcu() call.

3. Third, more important issue is related to outgoing cpu. Checking
only for __num_online_cpus with preemotion disabled isn't sufficient
for RCU, as, on completion of CPUHP_TEARDOWN_CPU stop machine (which
clears outgoing cpu from __num_online_cpus, the CPU switches to idle
task. So, checking only for __num_online_cpus does not consider
RCU read side sections in scheduler code (before switching to idle
task) and any potential read side sections in idle task, before final
RCU-quiesce entry into cpuhp_report_idle_dead() -> rcu_report_dead().

To handle these issues, add a new rcu_state member n_online_cpus, to
keep account of the current number of online cpus. The atomic updates
to this counter from rcu_report_dead() and rcu_cpu_starting() and
the added read/write memory ordering semantics ensure that
synchronize_rcu() fast path waits for all read side sections, where
incoming/outgoing cpus are considered online, for RCU i.e. after
rcu_cpu_starting() and before rcu_report_dead().

Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
---

Below is the reproducer for issue described in point 3; this snippet
is based on klitmus generated test, which is modified to sample reads
from idle thread:

static void code0(int* x) {
WRITE_ONCE(*x, 1);
idle_ctr = 0;
smp_mb();
mdelay(10);
WRITE_ONCE(*x, 1);
idle_ctr = 1;
synchronize_rcu();
WRITE_ONCE(*x, 2);
idle_ctr = 2;

}

static int thread0(void *_p) {
int _j, _i;
ctx_t *_a = (ctx_t *)_p;

smp_mb();
for (_j = 0 ; _j < stride ; _j++) {
for (_i = _j ; _i < size ; _i += stride) {
while (idle_wait1) {
cpu_relax();
cond_resched();
}
code0(&_a->x[_i]);
smp_mb();
get_online_cpus();
idle_wait1 = true;
put_online_cpus();
}
}
atomic_inc(&done);
smp_mb();
wake_up(wq);
smp_mb();
do_exit(0);
}


static void code1(int* x,int* out_1_r1,int* out_1_r0) {

int r0; int r1;

r0 = READ_ONCE(idle_ctr_snap1);
r1 = READ_ONCE(idle_ctr_snap2);

*out_1_r1 = (int)r1;
*out_1_r0 = (int)r0;
}

static int thread1(void *_p) {
ctx_t *_a = (ctx_t *)_p;
int _j, _i;

smp_mb();
for (_j = 0 ; _j < stride ; _j++) {
for (_i = _j ; _i < size ; _i += stride) {
while (idle_wait2) {
cpu_relax();
cond_resched();
}
get_online_cpus();
code1(&_a->x[_i],&_a->out_1_r1[_i],&_a->out_1_r0[_i]);
smp_mb();
idle_wait2 = true;
put_online_cpus();
}
}
atomic_inc(&done);
smp_mb();
wake_up(wq);
smp_mb();
do_exit(0);
}

Idle thread snippet:

if (cpu_is_offline(cpu)) {
smp_mb();
idle_wait1 = false;
mdelay(8);
smp_mb();
rcu_read_lock();
idle_ctr_snap1 = idle_ctr;
mdelay(40);
smp_rmb();
idle_ctr_snap2 = idle_ctr;
rcu_read_unlock();
smp_mb();
idle_wait2 = false;
tick_nohz_idle_stop_tick();
cpuhp_report_idle_dead();
arch_cpu_idle_dead();

kernel/rcu/tree.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
kernel/rcu/tree.h | 1 +
2 files changed, 66 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2424e2a..33493f0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3609,9 +3609,59 @@ static int rcu_blocking_is_gp(void)
if (IS_ENABLED(CONFIG_PREEMPTION))
return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
might_sleep(); /* Check for RCU read-side critical section. */
+ /*
+ * a = p
+ * a = NULL
+ * synchronize_rcu()
+ * rcu_blocking_is_gp()
+ * num_online_cpus()
+ * atomic_read(&__num_online_cpus)
+ * kfree(p);
+ *
+ * - VS -
+ *
+ * __cpu_up()
+ * set_cpu_online(cpu)
+ * atomic_inc(&__num_online_cpus)
+ * rcu_read_lock()
+ * rcu_dereference(a) (a == p)
+ * rcu_read_unlock()
+ *
+ * rcu_blocking_is_gp() must observe atomic_inc(&__num_online_cpus),
+ * in order to ensure that, RCU read side critical section on new
+ * online cpu, either start after synchronize_rcu()'s GP starts or
+ * it completes before synchronize_rcu() returns.
+ *
+ * However, atomic_read(&__num_online_cpus) does not ensure that.
+ *
+ * Essentially, below condition exist:
+ *
+ * {
+ * int x = 0;
+ * atomic_t numonline = ATOMIC_INIT(1);
+ * }
+ *
+ * P0(int *x, atomic_t *numonline) P1(int *x, atomic_t *numonline)
+ * { {
+ * int r0; int r0; int r1;
+ * WRITE_ONCE(*x, 1); atomic_inc(numonline);
+ * r0 = atomic_read(numonline); rcu_read_lock();
+ * if (r0 == 2) { r0 = READ_ONCE(*x);
+ * synchronize_rcu(); smp_rmb();
+ * } r1 = READ_ONCE(*x);
+ * WRITE_ONCE(*x, 2); rcu_read_unlock();
+ * } }
+ *
+ * exists (1:r0=0 /\ 1:r1=2)
+ *
+ * atomic_add_return(0, &rcu_state.n_online_cpus) and corresponding
+ * atomic_inc(&rcu_state.n_online_cpus) in rcu_cpu_starting() corrects
+ * this ordering issue.
+ */
preempt_disable();
ret = num_online_cpus() <= 1;
preempt_enable();
+ ret = ret && (atomic_add_return(0, &rcu_state.n_online_cpus) <= 1);
return ret;
}

@@ -3655,6 +3705,11 @@ void synchronize_rcu(void)
lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_rcu() in RCU read-side critical section");
if (rcu_blocking_is_gp())
+ /*
+ * atomic_add_return() in rcu_blocking_is_gp () provides
+ * full memory barrier ordering with any rcu section after
+ * synchronize_rcu() call.
+ */
return;
if (rcu_gp_is_expedited())
synchronize_rcu_expedited();
@@ -4086,6 +4141,10 @@ void rcu_cpu_starting(unsigned int cpu)
mask = rdp->grpmask;
raw_spin_lock_irqsave_rcu_node(rnp, flags);
WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
+ /* Order with access of n_online_cpus in rcu_blocking_is_gp */
+ atomic_inc(&rcu_state.n_online_cpus);
+ /* Order with rcu-side usages after this */
+ smp_mb__after_atomic();
newcpu = !(rnp->expmaskinitnext & mask);
rnp->expmaskinitnext |= mask;
/* Allow lockless access for expedited grace periods. */
@@ -4138,6 +4197,12 @@ void rcu_report_dead(unsigned int cpu)
raw_spin_lock_irqsave_rcu_node(rnp, flags);
}
WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask);
+ /*
+ * Order with access of n_online_cpus in rcu_blocking_is_gp().
+ * Release semantics ensures that RCU read sections before it
+ * are observed by rcu_blocking_is_gp().
+ */
+ atomic_dec_return_release(&rcu_state.n_online_cpus);
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
raw_spin_unlock(&rcu_state.ofl_lock);

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e4f66b8..4d9a9c0 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -298,6 +298,7 @@ struct rcu_state {
/* Hierarchy levels (+1 to */
/* shut bogus gcc warning) */
int ncpus; /* # CPUs seen so far. */
+ atomic_t n_online_cpus; /* # CPUs online for RCU. */

/* The following fields are guarded by the root rcu_node's lock. */

--
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a Linux Foundation Collaborative Project
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