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56222b212e
When PREEMPT_RT_FULL does the spinlock -> rt_mutex substitution the PI
chain code will (falsely) report a deadlock and BUG.
The problem is that it hold hb->lock (now an rt_mutex) while doing
task_blocks_on_rt_mutex on the futex's pi_state::rtmutex. This, when
interleaved just right with futex_unlock_pi() leads it to believe to see an
AB-BA deadlock.
Task1 (holds rt_mutex, Task2 (does FUTEX_LOCK_PI)
does FUTEX_UNLOCK_PI)
lock hb->lock
lock rt_mutex (as per start_proxy)
lock hb->lock
Which is a trivial AB-BA.
It is not an actual deadlock, because it won't be holding hb->lock by the
time it actually blocks on the rt_mutex, but the chainwalk code doesn't
know that and it would be a nightmare to handle this gracefully.
To avoid this problem, do the same as in futex_unlock_pi() and drop
hb->lock after acquiring wait_lock. This still fully serializes against
futex_unlock_pi(), since adding to the wait_list does the very same lock
dance, and removing it holds both locks.
Aside of solving the RT problem this makes the lock and unlock mechanism
symetric and reduces the hb->lock held time.
Reported-and-tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: juri.lelli@arm.com
Cc: xlpang@redhat.com
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: jdesfossez@efficios.com
Cc: dvhart@infradead.org
Cc: bristot@redhat.com
Link: http://lkml.kernel.org/r/20170322104152.161341537@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
133 lines
3.6 KiB
C
133 lines
3.6 KiB
C
/*
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* RT Mutexes: blocking mutual exclusion locks with PI support
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*
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* started by Ingo Molnar and Thomas Gleixner:
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*
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* Copyright (C) 2004-2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
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* Copyright (C) 2006, Timesys Corp., Thomas Gleixner <tglx@timesys.com>
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*
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* This file contains the private data structure and API definitions.
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*/
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#ifndef __KERNEL_RTMUTEX_COMMON_H
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#define __KERNEL_RTMUTEX_COMMON_H
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#include <linux/rtmutex.h>
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#include <linux/sched/wake_q.h>
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/*
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* This is the control structure for tasks blocked on a rt_mutex,
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* which is allocated on the kernel stack on of the blocked task.
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*
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* @tree_entry: pi node to enqueue into the mutex waiters tree
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* @pi_tree_entry: pi node to enqueue into the mutex owner waiters tree
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* @task: task reference to the blocked task
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*/
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struct rt_mutex_waiter {
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struct rb_node tree_entry;
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struct rb_node pi_tree_entry;
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struct task_struct *task;
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struct rt_mutex *lock;
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#ifdef CONFIG_DEBUG_RT_MUTEXES
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unsigned long ip;
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struct pid *deadlock_task_pid;
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struct rt_mutex *deadlock_lock;
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#endif
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int prio;
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};
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/*
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* Various helpers to access the waiters-tree:
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*/
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static inline int rt_mutex_has_waiters(struct rt_mutex *lock)
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{
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return !RB_EMPTY_ROOT(&lock->waiters);
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}
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static inline struct rt_mutex_waiter *
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rt_mutex_top_waiter(struct rt_mutex *lock)
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{
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struct rt_mutex_waiter *w;
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w = rb_entry(lock->waiters_leftmost, struct rt_mutex_waiter,
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tree_entry);
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BUG_ON(w->lock != lock);
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return w;
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}
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static inline int task_has_pi_waiters(struct task_struct *p)
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{
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return !RB_EMPTY_ROOT(&p->pi_waiters);
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}
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static inline struct rt_mutex_waiter *
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task_top_pi_waiter(struct task_struct *p)
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{
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return rb_entry(p->pi_waiters_leftmost, struct rt_mutex_waiter,
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pi_tree_entry);
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}
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/*
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* lock->owner state tracking:
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*/
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#define RT_MUTEX_HAS_WAITERS 1UL
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static inline struct task_struct *rt_mutex_owner(struct rt_mutex *lock)
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{
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unsigned long owner = (unsigned long) READ_ONCE(lock->owner);
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return (struct task_struct *) (owner & ~RT_MUTEX_HAS_WAITERS);
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}
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/*
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* Constants for rt mutex functions which have a selectable deadlock
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* detection.
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*
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* RT_MUTEX_MIN_CHAINWALK: Stops the lock chain walk when there are
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* no further PI adjustments to be made.
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*
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* RT_MUTEX_FULL_CHAINWALK: Invoke deadlock detection with a full
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* walk of the lock chain.
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*/
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enum rtmutex_chainwalk {
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RT_MUTEX_MIN_CHAINWALK,
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RT_MUTEX_FULL_CHAINWALK,
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};
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/*
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* PI-futex support (proxy locking functions, etc.):
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*/
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extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);
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extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
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struct task_struct *proxy_owner);
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extern void rt_mutex_proxy_unlock(struct rt_mutex *lock,
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struct task_struct *proxy_owner);
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extern void rt_mutex_init_waiter(struct rt_mutex_waiter *waiter);
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extern int __rt_mutex_start_proxy_lock(struct rt_mutex *lock,
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struct rt_mutex_waiter *waiter,
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struct task_struct *task);
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extern int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
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struct rt_mutex_waiter *waiter,
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struct task_struct *task);
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extern int rt_mutex_wait_proxy_lock(struct rt_mutex *lock,
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struct hrtimer_sleeper *to,
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struct rt_mutex_waiter *waiter);
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extern bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
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struct rt_mutex_waiter *waiter);
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extern int rt_mutex_futex_trylock(struct rt_mutex *l);
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extern void rt_mutex_futex_unlock(struct rt_mutex *lock);
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extern bool __rt_mutex_futex_unlock(struct rt_mutex *lock,
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struct wake_q_head *wqh);
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extern void rt_mutex_adjust_prio(struct task_struct *task);
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#ifdef CONFIG_DEBUG_RT_MUTEXES
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# include "rtmutex-debug.h"
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#else
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# include "rtmutex.h"
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#endif
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#endif
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