[PATCH RT 14/19] Revert "rtmutex: Handle the various new futex race conditions"

From: zanussi
Date: Thu Aug 08 2019 - 15:54:09 EST

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From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

v4.14.137-rt65-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------

[ Upstream commit 9e0265c21af4d6388d47dcd5ce20f76ec3a2e468 ]

Drop the RT fixup, the futex code will be changed to avoid the need for
the workaround.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Tom Zanussi <zanussi@xxxxxxxxxx>
---
kernel/futex.c | 77 ++++++++---------------------------------
kernel/locking/rtmutex.c | 36 ++++---------------
kernel/locking/rtmutex_common.h | 2 --
3 files changed, 21 insertions(+), 94 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 07b148ad703a..cb7e212fba0f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2165,16 +2165,6 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
requeue_pi_wake_futex(this, &key2, hb2);
drop_count++;
continue;
- } else if (ret == -EAGAIN) {
- /*
- * Waiter was woken by timeout or
- * signal and has set pi_blocked_on to
- * PI_WAKEUP_INPROGRESS before we
- * tried to enqueue it on the rtmutex.
- */
- this->pi_state = NULL;
- put_pi_state(pi_state);
- continue;
} else if (ret) {
/*
* rt_mutex_start_proxy_lock() detected a
@@ -3253,7 +3243,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
struct hrtimer_sleeper timeout, *to = NULL;
struct futex_pi_state *pi_state = NULL;
struct rt_mutex_waiter rt_waiter;
- struct futex_hash_bucket *hb, *hb2;
+ struct futex_hash_bucket *hb;
union futex_key key2 = FUTEX_KEY_INIT;
struct futex_q q = futex_q_init;
int res, ret;
@@ -3311,55 +3301,20 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
/* Queue the futex_q, drop the hb lock, wait for wakeup. */
futex_wait_queue_me(hb, &q, to);

- /*
- * On RT we must avoid races with requeue and trying to block
- * on two mutexes (hb->lock and uaddr2's rtmutex) by
- * serializing access to pi_blocked_on with pi_lock.
- */
- raw_spin_lock_irq(&current->pi_lock);
- if (current->pi_blocked_on) {
- /*
- * We have been requeued or are in the process of
- * being requeued.
- */
- raw_spin_unlock_irq(&current->pi_lock);
- } else {
- /*
- * Setting pi_blocked_on to PI_WAKEUP_INPROGRESS
- * prevents a concurrent requeue from moving us to the
- * uaddr2 rtmutex. After that we can safely acquire
- * (and possibly block on) hb->lock.
- */
- current->pi_blocked_on = PI_WAKEUP_INPROGRESS;
- raw_spin_unlock_irq(&current->pi_lock);
-
- spin_lock(&hb->lock);
-
- /*
- * Clean up pi_blocked_on. We might leak it otherwise
- * when we succeeded with the hb->lock in the fast
- * path.
- */
- raw_spin_lock_irq(&current->pi_lock);
- current->pi_blocked_on = NULL;
- raw_spin_unlock_irq(&current->pi_lock);
-
- ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
- spin_unlock(&hb->lock);
- if (ret)
- goto out_put_keys;
- }
+ spin_lock(&hb->lock);
+ ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
+ spin_unlock(&hb->lock);
+ if (ret)
+ goto out_put_keys;

/*
- * In order to be here, we have either been requeued, are in
- * the process of being requeued, or requeue successfully
- * acquired uaddr2 on our behalf. If pi_blocked_on was
- * non-null above, we may be racing with a requeue. Do not
- * rely on q->lock_ptr to be hb2->lock until after blocking on
- * hb->lock or hb2->lock. The futex_requeue dropped our key1
- * reference and incremented our key2 reference count.
+ * In order for us to be here, we know our q.key == key2, and since
+ * we took the hb->lock above, we also know that futex_requeue() has
+ * completed and we no longer have to concern ourselves with a wakeup
+ * race with the atomic proxy lock acquisition by the requeue code. The
+ * futex_requeue dropped our key1 reference and incremented our key2
+ * reference count.
*/
- hb2 = hash_futex(&key2);

/* Check if the requeue code acquired the second futex for us. */
if (!q.rt_waiter) {
@@ -3368,8 +3323,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
* did a lock-steal - fix up the PI-state in that case.
*/
if (q.pi_state && (q.pi_state->owner != current)) {
- spin_lock(&hb2->lock);
- BUG_ON(&hb2->lock != q.lock_ptr);
+ spin_lock(q.lock_ptr);
ret = fixup_pi_state_owner(uaddr2, &q, current);
if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current) {
pi_state = q.pi_state;
@@ -3380,7 +3334,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
* the requeue_pi() code acquired for us.
*/
put_pi_state(q.pi_state);
- spin_unlock(&hb2->lock);
+ spin_unlock(q.lock_ptr);
}
} else {
struct rt_mutex *pi_mutex;
@@ -3394,8 +3348,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
pi_mutex = &q.pi_state->pi_mutex;
ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);

- spin_lock(&hb2->lock);
- BUG_ON(&hb2->lock != q.lock_ptr);
+ spin_lock(q.lock_ptr);
if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
ret = 0;

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 62914dde3f1c..e1497623780b 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -142,11 +142,6 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
WRITE_ONCE(*p, owner & ~RT_MUTEX_HAS_WAITERS);
}

-static int rt_mutex_real_waiter(struct rt_mutex_waiter *waiter)
-{
- return waiter && waiter != PI_WAKEUP_INPROGRESS;
-}
-
/*
* We can speed up the acquire/release, if there's no debugging state to be
* set up.
@@ -420,8 +415,7 @@ int max_lock_depth = 1024;

static inline struct rt_mutex *task_blocked_on_lock(struct task_struct *p)
{
- return rt_mutex_real_waiter(p->pi_blocked_on) ?
- p->pi_blocked_on->lock : NULL;
+ return p->pi_blocked_on ? p->pi_blocked_on->lock : NULL;
}

/*
@@ -557,7 +551,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task,
* reached or the state of the chain has changed while we
* dropped the locks.
*/
- if (!rt_mutex_real_waiter(waiter))
+ if (!waiter)
goto out_unlock_pi;

/*
@@ -1340,22 +1334,6 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
return -EDEADLK;

raw_spin_lock(&task->pi_lock);
- /*
- * In the case of futex requeue PI, this will be a proxy
- * lock. The task will wake unaware that it is enqueueed on
- * this lock. Avoid blocking on two locks and corrupting
- * pi_blocked_on via the PI_WAKEUP_INPROGRESS
- * flag. futex_wait_requeue_pi() sets this when it wakes up
- * before requeue (due to a signal or timeout). Do not enqueue
- * the task if PI_WAKEUP_INPROGRESS is set.
- */
- if (task != current && task->pi_blocked_on == PI_WAKEUP_INPROGRESS) {
- raw_spin_unlock(&task->pi_lock);
- return -EAGAIN;
- }
-
- BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on));
-
waiter->task = task;
waiter->lock = lock;
waiter->prio = task->prio;
@@ -1379,7 +1357,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
rt_mutex_enqueue_pi(owner, waiter);

rt_mutex_adjust_prio(owner);
- if (rt_mutex_real_waiter(owner->pi_blocked_on))
+ if (owner->pi_blocked_on)
chain_walk = 1;
} else if (rt_mutex_cond_detect_deadlock(waiter, chwalk)) {
chain_walk = 1;
@@ -1479,7 +1457,7 @@ static void remove_waiter(struct rt_mutex *lock,
{
bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
struct task_struct *owner = rt_mutex_owner(lock);
- struct rt_mutex *next_lock = NULL;
+ struct rt_mutex *next_lock;

lockdep_assert_held(&lock->wait_lock);

@@ -1505,8 +1483,7 @@ static void remove_waiter(struct rt_mutex *lock,
rt_mutex_adjust_prio(owner);

/* Store the lock on which owner is blocked or NULL */
- if (rt_mutex_real_waiter(owner->pi_blocked_on))
- next_lock = task_blocked_on_lock(owner);
+ next_lock = task_blocked_on_lock(owner);

raw_spin_unlock(&owner->pi_lock);

@@ -1542,8 +1519,7 @@ void rt_mutex_adjust_pi(struct task_struct *task)
raw_spin_lock_irqsave(&task->pi_lock, flags);

waiter = task->pi_blocked_on;
- if (!rt_mutex_real_waiter(waiter) ||
- rt_mutex_waiter_equal(waiter, task_to_waiter(task))) {
+ if (!waiter || rt_mutex_waiter_equal(waiter, task_to_waiter(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 53ca0242101a..2f6662d052d6 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -131,8 +131,6 @@ enum rtmutex_chainwalk {
/*
* PI-futex support (proxy locking functions, etc.):
*/
-#define PI_WAKEUP_INPROGRESS ((struct rt_mutex_waiter *) 1)
-
extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);
extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
struct task_struct *proxy_owner);
--
2.14.1

Next message: zanussi: "[PATCH RT 10/19] pci/switchtec: fix stream_open.cocci warnings"
Previous message: Daniel Vetter: "Re: [PATCH 01/26] drm/dp_mst: Move link address dumping into a function"
In reply to: zanussi: "[PATCH RT 18/19] mm/zswap: Do not disable preemption in zswap_frontswap_store()"
Next in thread: zanussi: "[PATCH RT 10/19] pci/switchtec: fix stream_open.cocci warnings"
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