By changing futex_lock_pi() to use rt_mutex_*_proxy_lock() all wait_list modifications are done under both hb->lock and wait_lock. This closes the obvious interleave pattern between futex_lock_pi() and futex_unlock_pi(), but not entirely so. See below: Before: futex_lock_pi() futex_unlock_pi() unlock hb->lock lock hb->lock unlock hb->lock lock rt_mutex->wait_lock unlock rt_mutex_wait_lock -EAGAIN lock rt_mutex->wait_lock list_add unlock rt_mutex->wait_lock schedule() lock rt_mutex->wait_lock list_del unlock rt_mutex->wait_lock <idem> -EAGAIN lock hb->lock After: futex_lock_pi() futex_unlock_pi() lock hb->lock lock rt_mutex->wait_lock list_add unlock rt_mutex->wait_lock unlock hb->lock schedule() lock hb->lock unlock hb->lock lock hb->lock lock rt_mutex->wait_lock list_del unlock rt_mutex->wait_lock lock rt_mutex->wait_lock unlock rt_mutex_wait_lock -EAGAIN unlock hb->lock It does however solve the earlier starvation/live-lock scenario which got introduced with the -EAGAIN since unlike the before scenario; where the -EAGAIN happens while futex_unlock_pi() doesn't hold any locks; in the after scenario it happens while futex_unlock_pi() actually holds a lock, and then it is serialized on that lock.
Showing the changes to kernel/locking (2 of 3 files). Show all
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| @@ -1492,19 +1492,6 @@ int __sched rt_mutex_lock_interruptible(struct rt_mutex *lock) · 1491 unmodified lines | ||
| 1492 | 1492 | } |
| 1493 | 1493 | EXPORT_SYMBOL_GPL(rt_mutex_lock_interruptible); |
| 1494 | 1494 | |
| 1495 | −/* | |
| 1496 | − * Futex variant with full deadlock detection. | |
| 1497 | − * Futex variants must not use the fast-path, see __rt_mutex_futex_unlock(). | |
| 1498 | − */ | |
| 1499 | −int __sched rt_mutex_timed_futex_lock(struct rt_mutex *lock, | |
| 1500 | − struct hrtimer_sleeper *timeout) | |
| 1501 | −{ | |
| 1502 | − might_sleep(); | |
| 1503 | − | |
| 1504 | − return rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, | |
| 1505 | − timeout, RT_MUTEX_FULL_CHAINWALK); | |
| 1506 | −} | |
| 1507 | − | |
| 1508 | 1495 | /* |
| 1509 | 1496 | * Futex variant, must not use fastpath. |
| 1510 | 1497 | */ |
| @@ -1782,12 +1769,6 @@ int rt_mutex_wait_proxy_lock(struct rt_mutex *lock, · 271 unmodified lines | ||
| 1782 | 1769 | /* sleep on the mutex */ |
| 1783 | 1770 | ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter); |
| 1784 | 1771 | |
| 1785 | − /* | |
| 1786 | − * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might | |
| 1787 | − * have to fix that up. | |
| 1788 | − */ | |
| 1789 | − fixup_rt_mutex_waiters(lock); | |
| 1790 | − | |
| 1791 | 1772 | raw_spin_unlock_irq(&lock->wait_lock); |
| 1792 | 1773 | |
| 1793 | 1774 | return ret; |
| @@ -1827,6 +1808,13 @@ bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock, · 33 unmodified lines | ||
| 1827 | 1808 | fixup_rt_mutex_waiters(lock); |
| 1828 | 1809 | cleanup = true; |
| 1829 | 1810 | } |
| 1811 | + | |
| 1812 | + /* | |
| 1813 | + * try_to_take_rt_mutex() sets the waiter bit unconditionally. We might | |
| 1814 | + * have to fix that up. | |
| 1815 | + */ | |
| 1816 | + fixup_rt_mutex_waiters(lock); | |
| 1817 | + | |
| 1830 | 1818 | raw_spin_unlock_irq(&lock->wait_lock); |
| 1831 | 1819 | |
| 1832 | 1820 | return cleanup; |
| @@ -113,7 +113,6 @@ extern int rt_mutex_wait_proxy_lock(struct rt_mutex *lock, · 112 unmodified lines | ||
| 113 | 113 | extern bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock, |
| 114 | 114 | struct rt_mutex_waiter *waiter); |
| 115 | 115 | |
| 116 | −extern int rt_mutex_timed_futex_lock(struct rt_mutex *l, struct hrtimer_sleeper *to); | |
| 117 | 116 | extern int rt_mutex_futex_trylock(struct rt_mutex *l); |
| 118 | 117 | |
| 119 | 118 | extern void rt_mutex_futex_unlock(struct rt_mutex *lock); |