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sched: Fix race between ttwu() and task_rq_lock() 

Thomas found that due to ttwu() changing a task's cpu without holding
the rq->lock, task_rq_lock() might end up locking the wrong rq.

Avoid this by serializing against TASK_WAKING.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1266241712.15770.420.camel@laptop>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Peter Zijlstra 2010-02-15 14:45:54 +01:00 committed by Thomas Gleixner
parent 9000f05c6d
commit 0970d2992d
1 changed files with 46 additions and 27 deletions
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73
kernel/sched.c
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@ -940,6 +940,19 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
}
#endif /* __ARCH_WANT_UNLOCKED_CTXSW */
/*
* Check whether the task is waking, we use this to synchronize against
* ttwu() so that task_cpu() reports a stable number.
*
* We need to make an exception for PF_STARTING tasks because the fork
* path might require task_rq_lock() to work, eg. it can call
* set_cpus_allowed_ptr() from the cpuset clone_ns code.
*/
static inline int task_is_waking(struct task_struct *p)
{
return unlikely((p->state == TASK_WAKING) && !(p->flags & PF_STARTING));
}
/*
* __task_rq_lock - lock the runqueue a given task resides on.
* Must be called interrupts disabled.
@ -947,10 +960,14 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
static inline struct rq *__task_rq_lock(struct task_struct *p)
__acquires(rq->lock)
{
struct rq *rq;
for (;;) {
struct rq *rq = task_rq(p);
while (task_is_waking(p))
cpu_relax();
rq = task_rq(p);
raw_spin_lock(&rq->lock);
if (likely(rq == task_rq(p)))
if (likely(rq == task_rq(p) && !task_is_waking(p)))
return rq;
raw_spin_unlock(&rq->lock);
}
@ -967,10 +984,12 @@ static struct rq *task_rq_lock(struct task_struct *p, unsigned long *flags)
struct rq *rq;
for (;;) {
while (task_is_waking(p))
cpu_relax();
local_irq_save(*flags);
rq = task_rq(p);
raw_spin_lock(&rq->lock);
if (likely(rq == task_rq(p)))
if (likely(rq == task_rq(p) && !task_is_waking(p)))
return rq;
raw_spin_unlock_irqrestore(&rq->lock, *flags);
}
@ -2408,14 +2427,27 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state,
__task_rq_unlock(rq);
cpu = select_task_rq(p, SD_BALANCE_WAKE, wake_flags);
if (cpu != orig_cpu)
if (cpu != orig_cpu) {
/*
* Since we migrate the task without holding any rq->lock,
* we need to be careful with task_rq_lock(), since that
* might end up locking an invalid rq.
*/
set_task_cpu(p, cpu);
}
rq = __task_rq_lock(p);
rq = cpu_rq(cpu);
raw_spin_lock(&rq->lock);
update_rq_clock(rq);
/*
* We migrated the task without holding either rq->lock, however
* since the task is not on the task list itself, nobody else
* will try and migrate the task, hence the rq should match the
* cpu we just moved it to.
*/
WARN_ON(task_cpu(p) != cpu);
WARN_ON(p->state != TASK_WAKING);
cpu = task_cpu(p);
#ifdef CONFIG_SCHEDSTATS
schedstat_inc(rq, ttwu_count);
@ -2647,7 +2679,7 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
{
unsigned long flags;
struct rq *rq;
int cpu __maybe_unused = get_cpu();
int cpu = get_cpu();
#ifdef CONFIG_SMP
/*
@ -2663,7 +2695,13 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
set_task_cpu(p, cpu);
#endif
rq = task_rq_lock(p, &flags);
/*
* Since the task is not on the rq and we still have TASK_WAKING set
* nobody else will migrate this task.
*/
rq = cpu_rq(cpu);
raw_spin_lock_irqsave(&rq->lock, flags);
BUG_ON(p->state != TASK_WAKING);
p->state = TASK_RUNNING;
update_rq_clock(rq);
@ -7156,27 +7194,8 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
struct rq *rq;
int ret = 0;
/*
* Since we rely on wake-ups to migrate sleeping tasks, don't change
* the ->cpus_allowed mask from under waking tasks, which would be
* possible when we change rq->lock in ttwu(), so synchronize against
* TASK_WAKING to avoid that.
*
* Make an exception for freshly cloned tasks, since cpuset namespaces
* might move the task about, we have to validate the target in
* wake_up_new_task() anyway since the cpu might have gone away.
*/
again:
while (p->state == TASK_WAKING && !(p->flags & PF_STARTING))
cpu_relax();
rq = task_rq_lock(p, &flags);
if (p->state == TASK_WAKING && !(p->flags & PF_STARTING)) {
task_rq_unlock(rq, &flags);
goto again;
}
if (!cpumask_intersects(new_mask, cpu_active_mask)) {
ret = -EINVAL;
goto out;