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rcu/nocb: Detect unsafe checks for offloaded rdp
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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.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
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>
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@intel-lab-lkp
Frederic Weisbecker authored and intel-lab-lkp committed Nov 13, 2020
1 parent eccda81 commit 4c0d56c
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Showing 2 changed files with 87 additions and 24 deletions.
21 changes: 11 additions & 10 deletions kernel/rcu/tree.c
View file
Expand Up @@ -154,6 +154,7 @@ 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;
Expand Down Expand Up @@ -1617,7 +1618,7 @@ 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);

Expand Down Expand Up @@ -2068,7 +2069,7 @@ 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);
Expand Down Expand Up @@ -2267,7 +2268,7 @@ 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());
Expand Down Expand Up @@ -2436,7 +2437,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
{
int div;
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;
Expand Down Expand Up @@ -2992,7 +2993,7 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
rcu_segcblist_n_cbs(&rdp->cblist));

/* 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);
Expand Down Expand Up @@ -3766,13 +3767,13 @@ 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;

Expand Down Expand Up @@ -3891,7 +3892,7 @@ 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,
Expand Down Expand Up @@ -4210,7 +4211,7 @@ 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. */

Expand All @@ -4228,7 +4229,7 @@ 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 {
Expand Down
90 changes: 76 additions & 14 deletions kernel/rcu/tree_plugin.h
View file
Expand Up @@ -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.
Expand Down Expand Up @@ -1256,7 +1318,7 @@ 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));
}

/*
Expand Down Expand Up @@ -1351,7 +1413,7 @@ 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;
}
Expand Down Expand Up @@ -1387,7 +1449,7 @@ 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. */
Expand Down Expand Up @@ -1428,7 +1490,7 @@ 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();
Expand Down Expand Up @@ -1554,7 +1616,7 @@ 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);
}
Expand All @@ -1565,7 +1627,7 @@ 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);
}
Expand All @@ -1578,7 +1640,7 @@ 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 {
Expand All @@ -1590,7 +1652,7 @@ 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);
}

Expand Down Expand Up @@ -1682,7 +1744,7 @@ 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)) {
Expand Down Expand Up @@ -1710,7 +1772,7 @@ 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);
Expand All @@ -1724,7 +1786,7 @@ 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));
Expand Down Expand Up @@ -1756,7 +1818,7 @@ 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. */
}
Expand Down Expand Up @@ -2378,7 +2440,7 @@ 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 {
Expand Down Expand Up @@ -2454,7 +2516,7 @@ 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 {
Expand Down

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