[PATCH tip/core/rcu 01/12] rcu/nocb: Detect unsafe checks for offloaded rdp

paulmck
2021-03-04T01:09:37Z

Provide CONFIG_PROVE_RCU sanity checks to ensure we are always reading
the offloaded state of an rdp in a safe and stable way and prevent from
its value to be changed under us. We must either hold the barrier mutex,
the cpu-hotplug lock (read or write) or the nocb lock.
Local non-preemptible reads are also safe. NOCB kthreads and timers have
their own means of synchronization against the offloaded state updaters.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree.c | 21 +++++------
kernel/rcu/tree_plugin.h | 90 ++++++++++++++++++++++++++++++++++++++++--------
2 files changed, 87 insertions(+), 24 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index da6f521..03503e2 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -156,6 +156,7 @@ static void invoke_rcu_core(void);
static void rcu_report_exp_rdp(struct rcu_data *rdp);
static void sync_sched_exp_online_cleanup(int cpu);
static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);
+static bool rcu_rdp_is_offloaded(struct rcu_data *rdp);

/* rcuc/rcub kthread realtime priority */
static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
@@ -1672,7 +1673,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
{
bool ret = false;
bool need_qs;
- const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
+ const bool offloaded = rcu_rdp_is_offloaded(rdp);

raw_lockdep_assert_held_rcu_node(rnp);

@@ -2128,7 +2129,7 @@ static void rcu_gp_cleanup(void)
needgp = true;
}
/* Advance CBs to reduce false positives below. */
- offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
+ offloaded = rcu_rdp_is_offloaded(rdp);
if ((offloaded || !rcu_accelerate_cbs(rnp, rdp)) && needgp) {
WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT);
WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
@@ -2327,7 +2328,7 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
unsigned long flags;
unsigned long mask;
bool needwake = false;
- const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
+ const bool offloaded = rcu_rdp_is_offloaded(rdp);
struct rcu_node *rnp;

WARN_ON_ONCE(rdp->cpu != smp_processor_id());
@@ -2497,7 +2498,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
int div;
bool __maybe_unused empty;
unsigned long flags;
- const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
+ const bool offloaded = rcu_rdp_is_offloaded(rdp);
struct rcu_head *rhp;
struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
long bl, count = 0;
@@ -3066,7 +3067,7 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));

/* Go handle any RCU core processing required. */
- if (unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
+ if (unlikely(rcu_rdp_is_offloaded(rdp))) {
__call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */
} else {
__call_rcu_core(rdp, head, flags);
@@ -3843,13 +3844,13 @@ static int rcu_pending(int user)
return 1;

/* Does this CPU have callbacks ready to invoke? */
- if (!rcu_segcblist_is_offloaded(&rdp->cblist) &&
+ if (!rcu_rdp_is_offloaded(rdp) &&
rcu_segcblist_ready_cbs(&rdp->cblist))
return 1;

/* Has RCU gone idle with this CPU needing another grace period? */
if (!gp_in_progress && rcu_segcblist_is_enabled(&rdp->cblist) &&
- !rcu_segcblist_is_offloaded(&rdp->cblist) &&
+ !rcu_rdp_is_offloaded(rdp) &&
!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
return 1;

@@ -3968,7 +3969,7 @@ void rcu_barrier(void)
for_each_possible_cpu(cpu) {
rdp = per_cpu_ptr(&rcu_data, cpu);
if (cpu_is_offline(cpu) &&
- !rcu_segcblist_is_offloaded(&rdp->cblist))
+ !rcu_rdp_is_offloaded(rdp))
continue;
if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
rcu_barrier_trace(TPS("OnlineQ"), cpu,
@@ -4291,7 +4292,7 @@ void rcutree_migrate_callbacks(int cpu)
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
bool needwake;

- if (rcu_segcblist_is_offloaded(&rdp->cblist) ||
+ if (rcu_rdp_is_offloaded(rdp) ||
rcu_segcblist_empty(&rdp->cblist))
return; /* No callbacks to migrate. */

@@ -4309,7 +4310,7 @@ void rcutree_migrate_callbacks(int cpu)
rcu_segcblist_disable(&rdp->cblist);
WARN_ON_ONCE(rcu_segcblist_empty(&my_rdp->cblist) !=
!rcu_segcblist_n_cbs(&my_rdp->cblist));
- if (rcu_segcblist_is_offloaded(&my_rdp->cblist)) {
+ if (rcu_rdp_is_offloaded(my_rdp)) {
raw_spin_unlock_rcu_node(my_rnp); /* irqs remain disabled. */
__call_rcu_nocb_wake(my_rdp, true, flags);
} else {
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 2d60377..cd513ea 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -16,8 +16,70 @@
#ifdef CONFIG_RCU_NOCB_CPU
static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
static bool __read_mostly rcu_nocb_poll; /* Offload kthread are to poll. */
+static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
+{
+ return lockdep_is_held(&rdp->nocb_lock);
+}
+
+static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
+{
+ /* Race on early boot between thread creation and assignment */
+ if (!rdp->nocb_cb_kthread || !rdp->nocb_gp_kthread)
+ return true;
+
+ if (current == rdp->nocb_cb_kthread || current == rdp->nocb_gp_kthread)
+ if (in_task())
+ return true;
+ return false;
+}
+
+static inline bool rcu_running_nocb_timer(struct rcu_data *rdp)
+{
+ return (timer_curr_running(&rdp->nocb_timer) && !in_irq());
+}
+#else
+static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
+{
+ return 0;
+}
+
+static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
+{
+ return false;
+}
+
+static inline bool rcu_running_nocb_timer(struct rcu_data *rdp)
+{
+ return false;
+}
+
#endif /* #ifdef CONFIG_RCU_NOCB_CPU */

+static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
+{
+ /*
+ * In order to read the offloaded state of an rdp is a safe
+ * and stable way and prevent from its value to be changed
+ * under us, we must either hold the barrier mutex, the cpu
+ * hotplug lock (read or write) or the nocb lock. Local
+ * non-preemptible reads are also safe. NOCB kthreads and
+ * timers have their own means of synchronization against the
+ * offloaded state updaters.
+ */
+ RCU_LOCKDEP_WARN(
+ !(lockdep_is_held(&rcu_state.barrier_mutex) ||
+ (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
+ rcu_lockdep_is_held_nocb(rdp) ||
+ (rdp == this_cpu_ptr(&rcu_data) &&
+ !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
+ rcu_current_is_nocb_kthread(rdp) ||
+ rcu_running_nocb_timer(rdp)),
+ "Unsafe read of RCU_NOCB offloaded state"
+ );
+
+ return rcu_segcblist_is_offloaded(&rdp->cblist);
+}
+
/*
* Check the RCU kernel configuration parameters and print informative
* messages about anything out of the ordinary.
@@ -1257,7 +1319,7 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{
*nextevt = KTIME_MAX;
return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
- !rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist);
+ !rcu_rdp_is_offloaded(this_cpu_ptr(&rcu_data));
}

/*
@@ -1352,7 +1414,7 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)

/* If no non-offloaded callbacks, RCU doesn't need the CPU. */
if (rcu_segcblist_empty(&rdp->cblist) ||
- rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) {
+ rcu_rdp_is_offloaded(rdp)) {
*nextevt = KTIME_MAX;
return 0;
}
@@ -1388,7 +1450,7 @@ static void rcu_prepare_for_idle(void)
int tne;

lockdep_assert_irqs_disabled();
- if (rcu_segcblist_is_offloaded(&rdp->cblist))
+ if (rcu_rdp_is_offloaded(rdp))
return;

/* Handle nohz enablement switches conservatively. */
@@ -1429,7 +1491,7 @@ static void rcu_cleanup_after_idle(void)
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);

lockdep_assert_irqs_disabled();
- if (rcu_segcblist_is_offloaded(&rdp->cblist))
+ if (rcu_rdp_is_offloaded(rdp))
return;
if (rcu_try_advance_all_cbs())
invoke_rcu_core();
@@ -1560,7 +1622,7 @@ static void rcu_nocb_bypass_unlock(struct rcu_data *rdp)
static void rcu_nocb_lock(struct rcu_data *rdp)
{
lockdep_assert_irqs_disabled();
- if (!rcu_segcblist_is_offloaded(&rdp->cblist))
+ if (!rcu_rdp_is_offloaded(rdp))
return;
raw_spin_lock(&rdp->nocb_lock);
}
@@ -1571,7 +1633,7 @@ static void rcu_nocb_lock(struct rcu_data *rdp)
*/
static void rcu_nocb_unlock(struct rcu_data *rdp)
{
- if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
+ if (rcu_rdp_is_offloaded(rdp)) {
lockdep_assert_irqs_disabled();
raw_spin_unlock(&rdp->nocb_lock);
}
@@ -1584,7 +1646,7 @@ static void rcu_nocb_unlock(struct rcu_data *rdp)
static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
unsigned long flags)
{
- if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
+ if (rcu_rdp_is_offloaded(rdp)) {
lockdep_assert_irqs_disabled();
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
} else {
@@ -1596,7 +1658,7 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
{
lockdep_assert_irqs_disabled();
- if (rcu_segcblist_is_offloaded(&rdp->cblist))
+ if (rcu_rdp_is_offloaded(rdp))
lockdep_assert_held(&rdp->nocb_lock);
}

@@ -1690,7 +1752,7 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
{
struct rcu_cblist rcl;

- WARN_ON_ONCE(!rcu_segcblist_is_offloaded(&rdp->cblist));
+ WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp));
rcu_lockdep_assert_cblist_protected(rdp);
lockdep_assert_held(&rdp->nocb_bypass_lock);
if (rhp && !rcu_cblist_n_cbs(&rdp->nocb_bypass)) {
@@ -1718,7 +1780,7 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
unsigned long j)
{
- if (!rcu_segcblist_is_offloaded(&rdp->cblist))
+ if (!rcu_rdp_is_offloaded(rdp))
return true;
rcu_lockdep_assert_cblist_protected(rdp);
rcu_nocb_bypass_lock(rdp);
@@ -1732,7 +1794,7 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
static void rcu_nocb_try_flush_bypass(struct rcu_data *rdp, unsigned long j)
{
rcu_lockdep_assert_cblist_protected(rdp);
- if (!rcu_segcblist_is_offloaded(&rdp->cblist) ||
+ if (!rcu_rdp_is_offloaded(rdp) ||
!rcu_nocb_bypass_trylock(rdp))
return;
WARN_ON_ONCE(!rcu_nocb_do_flush_bypass(rdp, NULL, j));
@@ -1764,7 +1826,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
unsigned long j = jiffies;
long ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);

- if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
+ if (!rcu_rdp_is_offloaded(rdp)) {
*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
return false; /* Not offloaded, no bypassing. */
}
@@ -2397,7 +2459,7 @@ int rcu_nocb_cpu_deoffload(int cpu)
}
mutex_lock(&rcu_state.barrier_mutex);
cpus_read_lock();
- if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
+ if (rcu_rdp_is_offloaded(rdp)) {
if (cpu_online(cpu))
ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
else
@@ -2472,7 +2534,7 @@ int rcu_nocb_cpu_offload(int cpu)

mutex_lock(&rcu_state.barrier_mutex);
cpus_read_lock();
- if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
+ if (!rcu_rdp_is_offloaded(rdp)) {
if (cpu_online(cpu))
ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
else
--
2.9.5