From: Xunlei Pang A crash happened while I was playing with deadline PI rtmutex. BUG: unable to handle kernel NULL pointer dereference at 0000000000000018 IP: [] 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: [] enqueue_task+0x2c/0x80 [] activate_task+0x23/0x30 [] pull_dl_task+0x1d5/0x260 [] pre_schedule_dl+0x16/0x20 [] __schedule+0xd3/0x900 [] schedule+0x29/0x70 [] __rt_mutex_slowlock+0x4b/0xc0 [] rt_mutex_slowlock+0xd1/0x190 [] rt_mutex_timed_lock+0x53/0x60 [] futex_lock_pi.isra.18+0x28c/0x390 [] do_futex+0x190/0x5b0 [] SyS_futex+0x80/0x180 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] 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 Cc: Ingo Molnar Cc: Juri Lelli Originally-From: Peter Zijlstra Signed-off-by: Xunlei Pang Signed-off-by: Peter Zijlstra (Intel) Link: http://lkml.kernel.org/r/1461659449-19497-2-git-send-email-xlpang@redhat.com --- 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;