Re: [RFC][PATCH 2/8] sched/rtmutex/deadline: Fix a PI crash for deadline tasks

From: Xunlei Pang
Date: Tue Jun 14 2016 - 08:54:09 EST

On 2016/06/14 at 18:21, Juri Lelli wrote:
> Hi,
>
> On 07/06/16 21:56, Peter Zijlstra wrote:
>> From: Xunlei Pang <xlpang@xxxxxxxxxx>
>>
>> A crash happened while I was playing with deadline PI rtmutex.
>>
>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
>> IP: [<ffffffff810eeb8f>] rt_mutex_get_top_task+0x1f/0x30
>> PGD 232a75067 PUD 230947067 PMD 0
>> Oops: 0000 [#1] SMP
>> CPU: 1 PID: 10994 Comm: a.out Not tainted
>>
>> Call Trace:
>> [<ffffffff810b658c>] enqueue_task+0x2c/0x80
>> [<ffffffff810ba763>] activate_task+0x23/0x30
>> [<ffffffff810d0ab5>] pull_dl_task+0x1d5/0x260
>> [<ffffffff810d0be6>] pre_schedule_dl+0x16/0x20
>> [<ffffffff8164e783>] __schedule+0xd3/0x900
>> [<ffffffff8164efd9>] schedule+0x29/0x70
>> [<ffffffff8165035b>] __rt_mutex_slowlock+0x4b/0xc0
>> [<ffffffff81650501>] rt_mutex_slowlock+0xd1/0x190
>> [<ffffffff810eeb33>] rt_mutex_timed_lock+0x53/0x60
>> [<ffffffff810ecbfc>] futex_lock_pi.isra.18+0x28c/0x390
>> [<ffffffff810ed8b0>] do_futex+0x190/0x5b0
>> [<ffffffff810edd50>] SyS_futex+0x80/0x180
>>
> This seems to be caused by the race condition you detail below between
> load balancing and PI code. I tried to reproduce the BUG on my box, but
> it looks hard to get. Do you have a reproducer I can give a try?
>
>> This is because rt_mutex_enqueue_pi() and rt_mutex_dequeue_pi()
>> are only protected by pi_lock when operating pi waiters, while
>> rt_mutex_get_top_task(), will access them with rq lock held but
>> not holding pi_lock.
>>
>> In order to tackle it, we introduce new "pi_top_task" pointer
>> cached in task_struct, and add new rt_mutex_update_top_task()
>> to update its value, it can be called by rt_mutex_setprio()
>> which held both owner's pi_lock and rq lock. Thus "pi_top_task"
>> can be safely accessed by enqueue_task_dl() under rq lock.
>>
>> [XXX this next section is unparsable]
> Yes, a bit hard to understand. However, am I correct in assuming this
> patch and the previous one should fix this problem? Or are there still
> other races causing issues?

Yes, these two patches can fix the problem.

>
>> One problem is when rt_mutex_adjust_prio()->...->rt_mutex_setprio(),
>> at that time rtmutex lock was released and owner was marked off,
>> this can cause "pi_top_task" dereferenced to be a running one(as it
>> can be falsely woken up by others before rt_mutex_setprio() is
>> made to update "pi_top_task"). We solve this by directly calling
>> __rt_mutex_adjust_prio() in mark_wakeup_next_waiter() which held
>> pi_lock and rtmutex lock, and remove rt_mutex_adjust_prio(). Since
>> now we moved the deboost point, in order to avoid current to be
>> preempted due to deboost earlier before wake_up_q(), we also moved
>> preempt_disable() before unlocking rtmutex.
>>
>> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Juri Lelli <juri.lelli@xxxxxxx>
>> Originally-From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxx>
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>> Link: http://lkml.kernel.org/r/1461659449-19497-2-git-send-email-xlpang@xxxxxxxxxx
> The idea of this fix makes sense to me. But, I would like to be able to
> see the BUG and test the fix. What I have is a test in which I create N
> DEADLINE workers that share a PI mutex. They get migrated around and
> seem to stress PI code. But I couldn't hit the BUG yet. Maybe I let it
> run for some more time.

You can use this reproducer attached(gcc crash_deadline_pi.c -lpthread -lrt ).
Start multiple instances, then it will hit the bug very soon.

Regards,
Xunlei

>
> Best,
>
> - Juri
>
>> ---
>>
>> include/linux/init_task.h | 1
>> include/linux/sched.h | 2 +
>> include/linux/sched/rt.h | 1
>> kernel/fork.c | 1
>> kernel/locking/rtmutex.c | 65 +++++++++++++++++++---------------------------
>> kernel/sched/core.c | 2 +
>> 6 files changed, 34 insertions(+), 38 deletions(-)
>>
>> --- a/include/linux/init_task.h
>> +++ b/include/linux/init_task.h
>> @@ -162,6 +162,7 @@ extern struct task_group root_task_group
>> #ifdef CONFIG_RT_MUTEXES
>> # define INIT_RT_MUTEXES(tsk) \
>> .pi_waiters = RB_ROOT, \
>> + .pi_top_task = NULL, \
>> .pi_waiters_leftmost = NULL,
>> #else
>> # define INIT_RT_MUTEXES(tsk)
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1681,6 +1681,8 @@ struct task_struct {
>> /* PI waiters blocked on a rt_mutex held by this task */
>> struct rb_root pi_waiters;
>> struct rb_node *pi_waiters_leftmost;
>> + /* Updated under owner's pi_lock and rq lock */
>> + struct task_struct *pi_top_task;
>> /* Deadlock detection and priority inheritance handling */
>> struct rt_mutex_waiter *pi_blocked_on;
>> #endif
>> --- a/include/linux/sched/rt.h
>> +++ b/include/linux/sched/rt.h
>> @@ -19,6 +19,7 @@ static inline int rt_task(struct task_st
>> extern int rt_mutex_getprio(struct task_struct *p);
>> extern void rt_mutex_setprio(struct task_struct *p, int prio);
>> extern int rt_mutex_get_effective_prio(struct task_struct *task, int newprio);
>> +extern void rt_mutex_update_top_task(struct task_struct *p);
>> extern struct task_struct *rt_mutex_get_top_task(struct task_struct *task);
>> extern void rt_mutex_adjust_pi(struct task_struct *p);
>> static inline bool tsk_is_pi_blocked(struct task_struct *tsk)
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1219,6 +1219,7 @@ static void rt_mutex_init_task(struct ta
>> #ifdef CONFIG_RT_MUTEXES
>> p->pi_waiters = RB_ROOT;
>> p->pi_waiters_leftmost = NULL;
>> + p->pi_top_task = NULL;
>> p->pi_blocked_on = NULL;
>> #endif
>> }
>> --- a/kernel/locking/rtmutex.c
>> +++ b/kernel/locking/rtmutex.c
>> @@ -256,6 +256,16 @@ rt_mutex_dequeue_pi(struct task_struct *
>> RB_CLEAR_NODE(&waiter->pi_tree_entry);
>> }
>>
>> +void rt_mutex_update_top_task(struct task_struct *p)
>> +{
>> + if (!task_has_pi_waiters(p)) {
>> + p->pi_top_task = NULL;
>> + return;
>> + }
>> +
>> + p->pi_top_task = task_top_pi_waiter(p)->task;
>> +}
>> +
>> /*
>> * Calculate task priority from the waiter tree priority
>> *
>> @@ -273,10 +283,7 @@ int rt_mutex_getprio(struct task_struct
>>
>> struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
>> {
>> - if (likely(!task_has_pi_waiters(task)))
>> - return NULL;
>> -
>> - return task_top_pi_waiter(task)->task;
>> + return task->pi_top_task;
>> }
>>
>> /*
>> @@ -285,12 +292,12 @@ struct task_struct *rt_mutex_get_top_tas
>> */
>> int rt_mutex_get_effective_prio(struct task_struct *task, int newprio)
>> {
>> - if (!task_has_pi_waiters(task))
>> + struct task_struct *top_task = rt_mutex_get_top_task(task);
>> +
>> + if (!top_task)
>> return newprio;
>>
>> - if (task_top_pi_waiter(task)->task->prio <= newprio)
>> - return task_top_pi_waiter(task)->task->prio;
>> - return newprio;
>> + return min(top_task->prio, newprio);
>> }
>>
>> /*
>> @@ -307,24 +314,6 @@ static void __rt_mutex_adjust_prio(struc
>> }
>>
>> /*
>> - * Adjust task priority (undo boosting). Called from the exit path of
>> - * rt_mutex_slowunlock() and rt_mutex_slowlock().
>> - *
>> - * (Note: We do this outside of the protection of lock->wait_lock to
>> - * allow the lock to be taken while or before we readjust the priority
>> - * of task. We do not use the spin_xx_mutex() variants here as we are
>> - * outside of the debug path.)
>> - */
>> -void rt_mutex_adjust_prio(struct task_struct *task)
>> -{
>> - unsigned long flags;
>> -
>> - raw_spin_lock_irqsave(&task->pi_lock, flags);
>> - __rt_mutex_adjust_prio(task);
>> - raw_spin_unlock_irqrestore(&task->pi_lock, flags);
>> -}
>> -
>> -/*
>> * Deadlock detection is conditional:
>> *
>> * If CONFIG_DEBUG_RT_MUTEXES=n, deadlock detection is only conducted
>> @@ -987,6 +976,7 @@ static void mark_wakeup_next_waiter(stru
>> * lock->wait_lock.
>> */
>> rt_mutex_dequeue_pi(current, waiter);
>> + __rt_mutex_adjust_prio(current);
>>
>> /*
>> * As we are waking up the top waiter, and the waiter stays
>> @@ -1325,6 +1315,16 @@ static bool __sched rt_mutex_slowunlock(
>> */
>> mark_wakeup_next_waiter(wake_q, lock);
>>
>> + /*
>> + * We should deboost before waking the top waiter task such that
>> + * we don't run two tasks with the 'same' priority. This however
>> + * can lead to prio-inversion if we would get preempted after
>> + * the deboost but before waking our high-prio task, hence the
>> + * preempt_disable before unlock. Pairs with preempt_enable() in
>> + * rt_mutex_postunlock();
>> + */
>> + preempt_disable();
>> +
>> raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
>>
>> /* check PI boosting */
>> @@ -1400,20 +1400,9 @@ rt_mutex_fastunlock(struct rt_mutex *loc
>> */
>> void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost)
>> {
>> - /*
>> - * We should deboost before waking the top waiter task such that
>> - * we don't run two tasks with the 'same' priority. This however
>> - * can lead to prio-inversion if we would get preempted after
>> - * the deboost but before waking our high-prio task, hence the
>> - * preempt_disable.
>> - */
>> - if (deboost) {
>> - preempt_disable();
>> - rt_mutex_adjust_prio(current);
>> - }
>> -
>> wake_up_q(wake_q);
>>
>> + /* Pairs with preempt_disable() in rt_mutex_slowunlock() */
>> if (deboost)
>> preempt_enable();
>> }
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -3568,6 +3568,8 @@ void rt_mutex_setprio(struct task_struct
>> goto out_unlock;
>> }
>>
>> + rt_mutex_update_top_task(p);
>> +
>> trace_sched_pi_setprio(p, prio);
>> oldprio = p->prio;
>>
>>
>>

#define _GNU_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <linux/unistd.h>
#include <linux/kernel.h>
#include <linux/types.h>
#include <sys/syscall.h>
#include <pthread.h>
#include <sched.h>
#include <time.h>

#define gettid() syscall(__NR_gettid)

#define SCHED_DEADLINE 6

/* XXX use the proper syscall numbers */
#ifdef __x86_64__
#define __NR_sched_setattr 314
#define __NR_sched_getattr 315
#endif

#ifdef __i386__
#define __NR_sched_setattr 351
#define __NR_sched_getattr 352
#endif

#ifdef __ppc64__
#define __NR_sched_setattr 355
#define __NR_sched_getattr 356
#endif

#ifdef __s390x__
#define __NR_sched_setattr 345
#define __NR_sched_getattr 346
#endif


static volatile int done;

struct sched_attr {
__u32 size;

__u32 sched_policy;
__u64 sched_flags;

/* SCHED_NORMAL, SCHED_BATCH */
__s32 sched_nice;

/* SCHED_FIFO, SCHED_RR */
__u32 sched_priority;

/* SCHED_DEADLINE (nsec) */
__u64 sched_runtime;
__u64 sched_deadline;
__u64 sched_period;
};

int sched_setattr(pid_t pid,
const struct sched_attr *attr,
unsigned int flags)
{
return syscall(__NR_sched_setattr, pid, attr, flags);
}

int sched_getattr(pid_t pid,
struct sched_attr *attr,
unsigned int size,
unsigned int flags)
{
return syscall(__NR_sched_getattr, pid, attr, size, flags);
}

pthread_mutex_t mutex_obj;
pthread_mutexattr_t mutex_attr;
int x = 0;

static int decide(void)
{
struct timespec ts;

clock_gettime(CLOCK_MONOTONIC, &ts);

if (ts.tv_nsec & 1)
return 1;
else
return 0;
}

static void mutex_init(void)
{
pthread_mutexattr_init(&mutex_attr);
pthread_mutexattr_setprotocol(&mutex_attr, PTHREAD_PRIO_INHERIT);
pthread_mutex_init(&mutex_obj, &mutex_attr);
}

static deadline_ndelay(unsigned int cnt)
{
unsigned int i;

for (i = 0; i < 10000 * cnt; i++);
}

void *run_deadline_special(void *data)
{
struct sched_attr attr;
int ret, take;
unsigned int flags = 0;

attr.size = sizeof(attr);
attr.sched_flags = 0;
attr.sched_nice = 0;
attr.sched_priority = 0;

/* This creates a 10ms/30ms reservation */
attr.sched_policy = SCHED_DEADLINE;
attr.sched_runtime = 100 * 1000 * 1000;
attr.sched_deadline = 200 * 1000 * 1000;
attr.sched_period = 300 * 1000 * 1000;

ret = sched_setattr(0, &attr, flags);
if (ret < 0) {
done = 0;
perror("sched_setattr");
exit(-1);
}

printf("special deadline thread started [%ld]\n", gettid());
while (!done) {
take = decide();
if (take)
pthread_mutex_lock(&mutex_obj);

x++;
deadline_ndelay((unsigned long) attr.sched_runtime % 7);

if (take)
pthread_mutex_unlock(&mutex_obj);
}

printf("special deadline thread dies [%ld]\n", gettid());
return NULL;
}

void *run_deadline(void *data)
{
struct sched_attr attr;
int ret, take;
unsigned int flags = 0;
static unsigned int delta = 0;


attr.size = sizeof(attr);
attr.sched_flags = 0;
attr.sched_nice = 0;
attr.sched_priority = 0;

/* This creates a 10ms/30ms reservation */
delta += 1000 * 1000 * 2;
attr.sched_policy = SCHED_DEADLINE;
attr.sched_runtime = 20 * 1000 * 1000 + delta;
attr.sched_deadline = 400 * 1000 * 1000;
attr.sched_period = 400 * 1000 * 1000;

ret = sched_setattr(0, &attr, flags);
if (ret < 0) {
done = 0;
perror("sched_setattr");
exit(-1);
}

printf("deadline thread started [%ld]\n", gettid());
while (!done) {
take = decide();
if (take)
pthread_mutex_lock(&mutex_obj);

x++;
deadline_ndelay((unsigned long) attr.sched_runtime % 7);

if (take)
pthread_mutex_unlock(&mutex_obj);
}

printf("deadline thread dies [%ld]\n", gettid());
return NULL;
}

#define THREAD_NUM 10

int main (int argc, char **argv)
{
pthread_t thread[THREAD_NUM];
int i;

mutex_init();

printf("main thread [%ld]\n", gettid());

for (i = 0; i < THREAD_NUM-1; i++)
pthread_create(&thread[i], NULL, run_deadline, NULL);

pthread_create(&thread[THREAD_NUM-1], NULL, run_deadline_special, NULL);

sleep(3600*300);

done = 1;
for (i = 0; i < THREAD_NUM; i++)
pthread_join(thread[i], NULL);

printf("main dies [%ld]\n", gettid());
return 0;
}