Commit 565790d2 authored by Peter Zijlstra's avatar Peter Zijlstra

sched: Fix balance_callback()

The intent of balance_callback() has always been to delay executing
balancing operations until the end of the current rq->lock section.
This is because balance operations must often drop rq->lock, and that
isn't safe in general.

However, as noted by Scott, there were a few holes in that scheme;
balance_callback() was called after rq->lock was dropped, which means
another CPU can interleave and touch the callback list.

Rework code to call the balance callbacks before dropping rq->lock
where possible, and otherwise splice the balance list onto a local
stack.

This guarantees that the balance list must be empty when we take
rq->lock. IOW, we'll only ever run our own balance callbacks.
Reported-by: default avatarScott Wood <swood@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarValentin Schneider <valentin.schneider@arm.com>
Reviewed-by: default avatarDaniel Bristot de Oliveira <bristot@redhat.com>
Link: https://lkml.kernel.org/r/20201023102346.203901269@infradead.org
parent a8b62fd0
......@@ -3485,6 +3485,69 @@ static inline void finish_task(struct task_struct *prev)
#endif
}
#ifdef CONFIG_SMP
static void do_balance_callbacks(struct rq *rq, struct callback_head *head)
{
void (*func)(struct rq *rq);
struct callback_head *next;
lockdep_assert_held(&rq->lock);
while (head) {
func = (void (*)(struct rq *))head->func;
next = head->next;
head->next = NULL;
head = next;
func(rq);
}
}
static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
{
struct callback_head *head = rq->balance_callback;
lockdep_assert_held(&rq->lock);
if (head)
rq->balance_callback = NULL;
return head;
}
static void __balance_callbacks(struct rq *rq)
{
do_balance_callbacks(rq, splice_balance_callbacks(rq));
}
static inline void balance_callbacks(struct rq *rq, struct callback_head *head)
{
unsigned long flags;
if (unlikely(head)) {
raw_spin_lock_irqsave(&rq->lock, flags);
do_balance_callbacks(rq, head);
raw_spin_unlock_irqrestore(&rq->lock, flags);
}
}
#else
static inline void __balance_callbacks(struct rq *rq)
{
}
static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
{
return NULL;
}
static inline void balance_callbacks(struct rq *rq, struct callback_head *head)
{
}
#endif
static inline void
prepare_lock_switch(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
{
......@@ -3510,6 +3573,7 @@ static inline void finish_lock_switch(struct rq *rq)
* prev into current:
*/
spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
__balance_callbacks(rq);
raw_spin_unlock_irq(&rq->lock);
}
......@@ -3651,43 +3715,6 @@ static struct rq *finish_task_switch(struct task_struct *prev)
return rq;
}
#ifdef CONFIG_SMP
/* rq->lock is NOT held, but preemption is disabled */
static void __balance_callback(struct rq *rq)
{
struct callback_head *head, *next;
void (*func)(struct rq *rq);
unsigned long flags;
raw_spin_lock_irqsave(&rq->lock, flags);
head = rq->balance_callback;
rq->balance_callback = NULL;
while (head) {
func = (void (*)(struct rq *))head->func;
next = head->next;
head->next = NULL;
head = next;
func(rq);
}
raw_spin_unlock_irqrestore(&rq->lock, flags);
}
static inline void balance_callback(struct rq *rq)
{
if (unlikely(rq->balance_callback))
__balance_callback(rq);
}
#else
static inline void balance_callback(struct rq *rq)
{
}
#endif
/**
* schedule_tail - first thing a freshly forked thread must call.
* @prev: the thread we just switched away from.
......@@ -3707,7 +3734,6 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
*/
rq = finish_task_switch(prev);
balance_callback(rq);
preempt_enable();
if (current->set_child_tid)
......@@ -4523,10 +4549,11 @@ static void __sched notrace __schedule(bool preempt)
rq = context_switch(rq, prev, next, &rf);
} else {
rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
rq_unlock_irq(rq, &rf);
}
balance_callback(rq);
rq_unpin_lock(rq, &rf);
__balance_callbacks(rq);
raw_spin_unlock_irq(&rq->lock);
}
}
void __noreturn do_task_dead(void)
......@@ -4937,9 +4964,11 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
out_unlock:
/* Avoid rq from going away on us: */
preempt_disable();
__task_rq_unlock(rq, &rf);
balance_callback(rq);
rq_unpin_lock(rq, &rf);
__balance_callbacks(rq);
raw_spin_unlock(&rq->lock);
preempt_enable();
}
#else
......@@ -5213,6 +5242,7 @@ static int __sched_setscheduler(struct task_struct *p,
int retval, oldprio, oldpolicy = -1, queued, running;
int new_effective_prio, policy = attr->sched_policy;
const struct sched_class *prev_class;
struct callback_head *head;
struct rq_flags rf;
int reset_on_fork;
int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
......@@ -5451,6 +5481,7 @@ static int __sched_setscheduler(struct task_struct *p,
/* Avoid rq from going away on us: */
preempt_disable();
head = splice_balance_callbacks(rq);
task_rq_unlock(rq, p, &rf);
if (pi) {
......@@ -5459,7 +5490,7 @@ static int __sched_setscheduler(struct task_struct *p,
}
/* Run balance callbacks after we've adjusted the PI chain: */
balance_callback(rq);
balance_callbacks(rq, head);
preempt_enable();
return 0;
......
......@@ -1221,6 +1221,9 @@ static inline void rq_pin_lock(struct rq *rq, struct rq_flags *rf)
rq->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
rf->clock_update_flags = 0;
#endif
#ifdef CONFIG_SMP
SCHED_WARN_ON(rq->balance_callback);
#endif
}
static inline void rq_unpin_lock(struct rq *rq, struct rq_flags *rf)
......
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