From: Peter Zijlstra peterz@infradead.org
[ Upstream commit f7853c34241807bb97673a5e97719123be39a09e ]
Henry reported that rt_mutex_adjust_prio_check() has an ordering problem and puts the lie to the comment in [7]. Sharing the sort key between lock->waiters and owner->pi_waiters *does* create problems, since unlike what the comment claims, holding [L] is insufficient.
Notably, consider:
A / \ M1 M2 | | B C
That is, task A owns both M1 and M2, B and C block on them. In this case a concurrent chain walk (B & C) will modify their resp. sort keys in [7] while holding M1->wait_lock and M2->wait_lock. So holding [L] is meaningless, they're different Ls.
This then gives rise to a race condition between [7] and [11], where the requeue of pi_waiters will observe an inconsistent tree order.
B C
(holds M1->wait_lock, (holds M2->wait_lock, holds B->pi_lock) holds A->pi_lock)
[7] waiter_update_prio(); ... [8] raw_spin_unlock(B->pi_lock); ... [10] raw_spin_lock(A->pi_lock);
[11] rt_mutex_enqueue_pi(); // observes inconsistent A->pi_waiters // tree order
Fixing this means either extending the range of the owner lock from [10-13] to [6-13], with the immediate problem that this means [6-8] hold both blocked and owner locks, or duplicating the sort key.
Since the locking in chain walk is horrible enough without having to consider pi_lock nesting rules, duplicate the sort key instead.
By giving each tree their own sort key, the above race becomes harmless, if C sees B at the old location, then B will correct things (if they need correcting) when it walks up the chain and reaches A.
Fixes: fb00aca47440 ("rtmutex: Turn the plist into an rb-tree") Reported-by: Henry Wu triangletrap12@gmail.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Thomas Gleixner tglx@linutronix.de Tested-by: Henry Wu triangletrap12@gmail.com Link: https://lkml.kernel.org/r/20230707161052.GF2883469%40hirez.programming.kicks... Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/locking/rtmutex.c | 170 +++++++++++++++++++++----------- kernel/locking/rtmutex_api.c | 2 +- kernel/locking/rtmutex_common.h | 47 ++++++--- kernel/locking/ww_mutex.h | 12 +-- 4 files changed, 155 insertions(+), 76 deletions(-)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 728f434de2bbf..21db0df0eb000 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -333,21 +333,43 @@ static __always_inline int __waiter_prio(struct task_struct *task) return prio; }
+/* + * Update the waiter->tree copy of the sort keys. + */ static __always_inline void waiter_update_prio(struct rt_mutex_waiter *waiter, struct task_struct *task) { - waiter->prio = __waiter_prio(task); - waiter->deadline = task->dl.deadline; + lockdep_assert_held(&waiter->lock->wait_lock); + lockdep_assert(RB_EMPTY_NODE(&waiter->tree.entry)); + + waiter->tree.prio = __waiter_prio(task); + waiter->tree.deadline = task->dl.deadline; +} + +/* + * Update the waiter->pi_tree copy of the sort keys (from the tree copy). + */ +static __always_inline void +waiter_clone_prio(struct rt_mutex_waiter *waiter, struct task_struct *task) +{ + lockdep_assert_held(&waiter->lock->wait_lock); + lockdep_assert_held(&task->pi_lock); + lockdep_assert(RB_EMPTY_NODE(&waiter->pi_tree.entry)); + + waiter->pi_tree.prio = waiter->tree.prio; + waiter->pi_tree.deadline = waiter->tree.deadline; }
/* - * Only use with rt_mutex_waiter_{less,equal}() + * Only use with rt_waiter_node_{less,equal}() */ +#define task_to_waiter_node(p) \ + &(struct rt_waiter_node){ .prio = __waiter_prio(p), .deadline = (p)->dl.deadline } #define task_to_waiter(p) \ - &(struct rt_mutex_waiter){ .prio = __waiter_prio(p), .deadline = (p)->dl.deadline } + &(struct rt_mutex_waiter){ .tree = *task_to_waiter_node(p) }
-static __always_inline int rt_mutex_waiter_less(struct rt_mutex_waiter *left, - struct rt_mutex_waiter *right) +static __always_inline int rt_waiter_node_less(struct rt_waiter_node *left, + struct rt_waiter_node *right) { if (left->prio < right->prio) return 1; @@ -364,8 +386,8 @@ static __always_inline int rt_mutex_waiter_less(struct rt_mutex_waiter *left, return 0; }
-static __always_inline int rt_mutex_waiter_equal(struct rt_mutex_waiter *left, - struct rt_mutex_waiter *right) +static __always_inline int rt_waiter_node_equal(struct rt_waiter_node *left, + struct rt_waiter_node *right) { if (left->prio != right->prio) return 0; @@ -385,7 +407,7 @@ static __always_inline int rt_mutex_waiter_equal(struct rt_mutex_waiter *left, static inline bool rt_mutex_steal(struct rt_mutex_waiter *waiter, struct rt_mutex_waiter *top_waiter) { - if (rt_mutex_waiter_less(waiter, top_waiter)) + if (rt_waiter_node_less(&waiter->tree, &top_waiter->tree)) return true;
#ifdef RT_MUTEX_BUILD_SPINLOCKS @@ -393,30 +415,30 @@ static inline bool rt_mutex_steal(struct rt_mutex_waiter *waiter, * Note that RT tasks are excluded from same priority (lateral) * steals to prevent the introduction of an unbounded latency. */ - if (rt_prio(waiter->prio) || dl_prio(waiter->prio)) + if (rt_prio(waiter->tree.prio) || dl_prio(waiter->tree.prio)) return false;
- return rt_mutex_waiter_equal(waiter, top_waiter); + return rt_waiter_node_equal(&waiter->tree, &top_waiter->tree); #else return false; #endif }
#define __node_2_waiter(node) \ - rb_entry((node), struct rt_mutex_waiter, tree_entry) + rb_entry((node), struct rt_mutex_waiter, tree.entry)
static __always_inline bool __waiter_less(struct rb_node *a, const struct rb_node *b) { struct rt_mutex_waiter *aw = __node_2_waiter(a); struct rt_mutex_waiter *bw = __node_2_waiter(b);
- if (rt_mutex_waiter_less(aw, bw)) + if (rt_waiter_node_less(&aw->tree, &bw->tree)) return 1;
if (!build_ww_mutex()) return 0;
- if (rt_mutex_waiter_less(bw, aw)) + if (rt_waiter_node_less(&bw->tree, &aw->tree)) return 0;
/* NOTE: relies on waiter->ww_ctx being set before insertion */ @@ -434,48 +456,58 @@ static __always_inline bool __waiter_less(struct rb_node *a, const struct rb_nod static __always_inline void rt_mutex_enqueue(struct rt_mutex_base *lock, struct rt_mutex_waiter *waiter) { - rb_add_cached(&waiter->tree_entry, &lock->waiters, __waiter_less); + lockdep_assert_held(&lock->wait_lock); + + rb_add_cached(&waiter->tree.entry, &lock->waiters, __waiter_less); }
static __always_inline void rt_mutex_dequeue(struct rt_mutex_base *lock, struct rt_mutex_waiter *waiter) { - if (RB_EMPTY_NODE(&waiter->tree_entry)) + lockdep_assert_held(&lock->wait_lock); + + if (RB_EMPTY_NODE(&waiter->tree.entry)) return;
- rb_erase_cached(&waiter->tree_entry, &lock->waiters); - RB_CLEAR_NODE(&waiter->tree_entry); + rb_erase_cached(&waiter->tree.entry, &lock->waiters); + RB_CLEAR_NODE(&waiter->tree.entry); }
-#define __node_2_pi_waiter(node) \ - rb_entry((node), struct rt_mutex_waiter, pi_tree_entry) +#define __node_2_rt_node(node) \ + rb_entry((node), struct rt_waiter_node, entry)
-static __always_inline bool -__pi_waiter_less(struct rb_node *a, const struct rb_node *b) +static __always_inline bool __pi_waiter_less(struct rb_node *a, const struct rb_node *b) { - return rt_mutex_waiter_less(__node_2_pi_waiter(a), __node_2_pi_waiter(b)); + return rt_waiter_node_less(__node_2_rt_node(a), __node_2_rt_node(b)); }
static __always_inline void rt_mutex_enqueue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter) { - rb_add_cached(&waiter->pi_tree_entry, &task->pi_waiters, __pi_waiter_less); + lockdep_assert_held(&task->pi_lock); + + rb_add_cached(&waiter->pi_tree.entry, &task->pi_waiters, __pi_waiter_less); }
static __always_inline void rt_mutex_dequeue_pi(struct task_struct *task, struct rt_mutex_waiter *waiter) { - if (RB_EMPTY_NODE(&waiter->pi_tree_entry)) + lockdep_assert_held(&task->pi_lock); + + if (RB_EMPTY_NODE(&waiter->pi_tree.entry)) return;
- rb_erase_cached(&waiter->pi_tree_entry, &task->pi_waiters); - RB_CLEAR_NODE(&waiter->pi_tree_entry); + rb_erase_cached(&waiter->pi_tree.entry, &task->pi_waiters); + RB_CLEAR_NODE(&waiter->pi_tree.entry); }
-static __always_inline void rt_mutex_adjust_prio(struct task_struct *p) +static __always_inline void rt_mutex_adjust_prio(struct rt_mutex_base *lock, + struct task_struct *p) { struct task_struct *pi_task = NULL;
+ lockdep_assert_held(&lock->wait_lock); + lockdep_assert(rt_mutex_owner(lock) == p); lockdep_assert_held(&p->pi_lock);
if (task_has_pi_waiters(p)) @@ -571,9 +603,14 @@ static __always_inline struct rt_mutex_base *task_blocked_on_lock(struct task_st * Chain walk basics and protection scope * * [R] refcount on task - * [P] task->pi_lock held + * [Pn] task->pi_lock held * [L] rtmutex->wait_lock held * + * Normal locking order: + * + * rtmutex->wait_lock + * task->pi_lock + * * Step Description Protected by * function arguments: * @task [R] @@ -588,27 +625,32 @@ static __always_inline struct rt_mutex_base *task_blocked_on_lock(struct task_st * again: * loop_sanity_check(); * retry: - * [1] lock(task->pi_lock); [R] acquire [P] - * [2] waiter = task->pi_blocked_on; [P] - * [3] check_exit_conditions_1(); [P] - * [4] lock = waiter->lock; [P] - * [5] if (!try_lock(lock->wait_lock)) { [P] try to acquire [L] - * unlock(task->pi_lock); release [P] + * [1] lock(task->pi_lock); [R] acquire [P1] + * [2] waiter = task->pi_blocked_on; [P1] + * [3] check_exit_conditions_1(); [P1] + * [4] lock = waiter->lock; [P1] + * [5] if (!try_lock(lock->wait_lock)) { [P1] try to acquire [L] + * unlock(task->pi_lock); release [P1] * goto retry; * } - * [6] check_exit_conditions_2(); [P] + [L] - * [7] requeue_lock_waiter(lock, waiter); [P] + [L] - * [8] unlock(task->pi_lock); release [P] + * [6] check_exit_conditions_2(); [P1] + [L] + * [7] requeue_lock_waiter(lock, waiter); [P1] + [L] + * [8] unlock(task->pi_lock); release [P1] * put_task_struct(task); release [R] * [9] check_exit_conditions_3(); [L] * [10] task = owner(lock); [L] * get_task_struct(task); [L] acquire [R] - * lock(task->pi_lock); [L] acquire [P] - * [11] requeue_pi_waiter(tsk, waiters(lock));[P] + [L] - * [12] check_exit_conditions_4(); [P] + [L] - * [13] unlock(task->pi_lock); release [P] + * lock(task->pi_lock); [L] acquire [P2] + * [11] requeue_pi_waiter(tsk, waiters(lock));[P2] + [L] + * [12] check_exit_conditions_4(); [P2] + [L] + * [13] unlock(task->pi_lock); release [P2] * unlock(lock->wait_lock); release [L] * goto again; + * + * Where P1 is the blocking task and P2 is the lock owner; going up one step + * the owner becomes the next blocked task etc.. + * +* */ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, enum rtmutex_chainwalk chwalk, @@ -756,7 +798,7 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, * enabled we continue, but stop the requeueing in the chain * walk. */ - if (rt_mutex_waiter_equal(waiter, task_to_waiter(task))) { + if (rt_waiter_node_equal(&waiter->tree, task_to_waiter_node(task))) { if (!detect_deadlock) goto out_unlock_pi; else @@ -764,13 +806,18 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, }
/* - * [4] Get the next lock + * [4] Get the next lock; per holding task->pi_lock we can't unblock + * and guarantee @lock's existence. */ lock = waiter->lock; /* * [5] We need to trylock here as we are holding task->pi_lock, * which is the reverse lock order versus the other rtmutex * operations. + * + * Per the above, holding task->pi_lock guarantees lock exists, so + * inverting this lock order is infeasible from a life-time + * perspective. */ if (!raw_spin_trylock(&lock->wait_lock)) { raw_spin_unlock_irq(&task->pi_lock); @@ -874,17 +921,18 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, * or * * DL CBS enforcement advancing the effective deadline. - * - * Even though pi_waiters also uses these fields, and that tree is only - * updated in [11], we can do this here, since we hold [L], which - * serializes all pi_waiters access and rb_erase() does not care about - * the values of the node being removed. */ waiter_update_prio(waiter, task);
rt_mutex_enqueue(lock, waiter);
- /* [8] Release the task */ + /* + * [8] Release the (blocking) task in preparation for + * taking the owner task in [10]. + * + * Since we hold lock->waiter_lock, task cannot unblock, even if we + * release task->pi_lock. + */ raw_spin_unlock(&task->pi_lock); put_task_struct(task);
@@ -908,7 +956,12 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, return 0; }
- /* [10] Grab the next task, i.e. the owner of @lock */ + /* + * [10] Grab the next task, i.e. the owner of @lock + * + * Per holding lock->wait_lock and checking for !owner above, there + * must be an owner and it cannot go away. + */ task = get_task_struct(rt_mutex_owner(lock)); raw_spin_lock(&task->pi_lock);
@@ -921,8 +974,9 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, * and adjust the priority of the owner. */ rt_mutex_dequeue_pi(task, prerequeue_top_waiter); + waiter_clone_prio(waiter, task); rt_mutex_enqueue_pi(task, waiter); - rt_mutex_adjust_prio(task); + rt_mutex_adjust_prio(lock, task);
} else if (prerequeue_top_waiter == waiter) { /* @@ -937,8 +991,9 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task, */ rt_mutex_dequeue_pi(task, waiter); waiter = rt_mutex_top_waiter(lock); + waiter_clone_prio(waiter, task); rt_mutex_enqueue_pi(task, waiter); - rt_mutex_adjust_prio(task); + rt_mutex_adjust_prio(lock, task); } else { /* * Nothing changed. No need to do any priority @@ -1154,6 +1209,7 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock, waiter->task = task; waiter->lock = lock; waiter_update_prio(waiter, task); + waiter_clone_prio(waiter, task);
/* Get the top priority waiter on the lock */ if (rt_mutex_has_waiters(lock)) @@ -1187,7 +1243,7 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock, rt_mutex_dequeue_pi(owner, top_waiter); rt_mutex_enqueue_pi(owner, waiter);
- rt_mutex_adjust_prio(owner); + rt_mutex_adjust_prio(lock, owner); if (owner->pi_blocked_on) chain_walk = 1; } else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) { @@ -1234,6 +1290,8 @@ static void __sched mark_wakeup_next_waiter(struct rt_wake_q_head *wqh, { struct rt_mutex_waiter *waiter;
+ lockdep_assert_held(&lock->wait_lock); + raw_spin_lock(¤t->pi_lock);
waiter = rt_mutex_top_waiter(lock); @@ -1246,7 +1304,7 @@ static void __sched mark_wakeup_next_waiter(struct rt_wake_q_head *wqh, * task unblocks. */ rt_mutex_dequeue_pi(current, waiter); - rt_mutex_adjust_prio(current); + rt_mutex_adjust_prio(lock, current);
/* * As we are waking up the top waiter, and the waiter stays @@ -1482,7 +1540,7 @@ static void __sched remove_waiter(struct rt_mutex_base *lock, if (rt_mutex_has_waiters(lock)) rt_mutex_enqueue_pi(owner, rt_mutex_top_waiter(lock));
- rt_mutex_adjust_prio(owner); + rt_mutex_adjust_prio(lock, owner);
/* Store the lock on which owner is blocked or NULL */ next_lock = task_blocked_on_lock(owner); diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c index cb9fdff76a8a3..a6974d0445930 100644 --- a/kernel/locking/rtmutex_api.c +++ b/kernel/locking/rtmutex_api.c @@ -459,7 +459,7 @@ void __sched rt_mutex_adjust_pi(struct task_struct *task) raw_spin_lock_irqsave(&task->pi_lock, flags);
waiter = task->pi_blocked_on; - if (!waiter || rt_mutex_waiter_equal(waiter, task_to_waiter(task))) { + if (!waiter || rt_waiter_node_equal(&waiter->tree, task_to_waiter_node(task))) { raw_spin_unlock_irqrestore(&task->pi_lock, flags); return; } diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h index c47e8361bfb5c..1162e07cdaea1 100644 --- a/kernel/locking/rtmutex_common.h +++ b/kernel/locking/rtmutex_common.h @@ -17,27 +17,44 @@ #include <linux/rtmutex.h> #include <linux/sched/wake_q.h>
+ +/* + * This is a helper for the struct rt_mutex_waiter below. A waiter goes in two + * separate trees and they need their own copy of the sort keys because of + * different locking requirements. + * + * @entry: rbtree node to enqueue into the waiters tree + * @prio: Priority of the waiter + * @deadline: Deadline of the waiter if applicable + * + * See rt_waiter_node_less() and waiter_*_prio(). + */ +struct rt_waiter_node { + struct rb_node entry; + int prio; + u64 deadline; +}; + /* * This is the control structure for tasks blocked on a rt_mutex, * which is allocated on the kernel stack on of the blocked task. * - * @tree_entry: pi node to enqueue into the mutex waiters tree - * @pi_tree_entry: pi node to enqueue into the mutex owner waiters tree + * @tree: node to enqueue into the mutex waiters tree + * @pi_tree: node to enqueue into the mutex owner waiters tree * @task: task reference to the blocked task * @lock: Pointer to the rt_mutex on which the waiter blocks * @wake_state: Wakeup state to use (TASK_NORMAL or TASK_RTLOCK_WAIT) - * @prio: Priority of the waiter - * @deadline: Deadline of the waiter if applicable * @ww_ctx: WW context pointer + * + * @tree is ordered by @lock->wait_lock + * @pi_tree is ordered by rt_mutex_owner(@lock)->pi_lock */ struct rt_mutex_waiter { - struct rb_node tree_entry; - struct rb_node pi_tree_entry; + struct rt_waiter_node tree; + struct rt_waiter_node pi_tree; struct task_struct *task; struct rt_mutex_base *lock; unsigned int wake_state; - int prio; - u64 deadline; struct ww_acquire_ctx *ww_ctx; };
@@ -105,7 +122,7 @@ static inline bool rt_mutex_waiter_is_top_waiter(struct rt_mutex_base *lock, { struct rb_node *leftmost = rb_first_cached(&lock->waiters);
- return rb_entry(leftmost, struct rt_mutex_waiter, tree_entry) == waiter; + return rb_entry(leftmost, struct rt_mutex_waiter, tree.entry) == waiter; }
static inline struct rt_mutex_waiter *rt_mutex_top_waiter(struct rt_mutex_base *lock) @@ -113,8 +130,10 @@ static inline struct rt_mutex_waiter *rt_mutex_top_waiter(struct rt_mutex_base * struct rb_node *leftmost = rb_first_cached(&lock->waiters); struct rt_mutex_waiter *w = NULL;
+ lockdep_assert_held(&lock->wait_lock); + if (leftmost) { - w = rb_entry(leftmost, struct rt_mutex_waiter, tree_entry); + w = rb_entry(leftmost, struct rt_mutex_waiter, tree.entry); BUG_ON(w->lock != lock); } return w; @@ -127,8 +146,10 @@ static inline int task_has_pi_waiters(struct task_struct *p)
static inline struct rt_mutex_waiter *task_top_pi_waiter(struct task_struct *p) { + lockdep_assert_held(&p->pi_lock); + return rb_entry(p->pi_waiters.rb_leftmost, struct rt_mutex_waiter, - pi_tree_entry); + pi_tree.entry); }
#define RT_MUTEX_HAS_WAITERS 1UL @@ -190,8 +211,8 @@ static inline void debug_rt_mutex_free_waiter(struct rt_mutex_waiter *waiter) static inline void rt_mutex_init_waiter(struct rt_mutex_waiter *waiter) { debug_rt_mutex_init_waiter(waiter); - RB_CLEAR_NODE(&waiter->pi_tree_entry); - RB_CLEAR_NODE(&waiter->tree_entry); + RB_CLEAR_NODE(&waiter->pi_tree.entry); + RB_CLEAR_NODE(&waiter->tree.entry); waiter->wake_state = TASK_NORMAL; waiter->task = NULL; } diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h index 56f139201f246..3ad2cc4823e59 100644 --- a/kernel/locking/ww_mutex.h +++ b/kernel/locking/ww_mutex.h @@ -96,25 +96,25 @@ __ww_waiter_first(struct rt_mutex *lock) struct rb_node *n = rb_first(&lock->rtmutex.waiters.rb_root); if (!n) return NULL; - return rb_entry(n, struct rt_mutex_waiter, tree_entry); + return rb_entry(n, struct rt_mutex_waiter, tree.entry); }
static inline struct rt_mutex_waiter * __ww_waiter_next(struct rt_mutex *lock, struct rt_mutex_waiter *w) { - struct rb_node *n = rb_next(&w->tree_entry); + struct rb_node *n = rb_next(&w->tree.entry); if (!n) return NULL; - return rb_entry(n, struct rt_mutex_waiter, tree_entry); + return rb_entry(n, struct rt_mutex_waiter, tree.entry); }
static inline struct rt_mutex_waiter * __ww_waiter_prev(struct rt_mutex *lock, struct rt_mutex_waiter *w) { - struct rb_node *n = rb_prev(&w->tree_entry); + struct rb_node *n = rb_prev(&w->tree.entry); if (!n) return NULL; - return rb_entry(n, struct rt_mutex_waiter, tree_entry); + return rb_entry(n, struct rt_mutex_waiter, tree.entry); }
static inline struct rt_mutex_waiter * @@ -123,7 +123,7 @@ __ww_waiter_last(struct rt_mutex *lock) struct rb_node *n = rb_last(&lock->rtmutex.waiters.rb_root); if (!n) return NULL; - return rb_entry(n, struct rt_mutex_waiter, tree_entry); + return rb_entry(n, struct rt_mutex_waiter, tree.entry); }
static inline void