From: Peter Zijlstra Date: Fri, 15 Sep 2023 17:19:44 +0200 Subject: [PATCH 7/7] futex/pi: Fix recursive rt_mutex waiter state Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.6/older/patches-6.6.7-rt18.tar.xz Some new assertions pointed out that the existing code has nested rt_mutex wait state in the futex code. Specifically, the futex_lock_pi() cancel case uses spin_lock() while there still is a rt_waiter enqueued for this task, resulting in a state where there are two waiters for the same task (and task_struct::pi_blocked_on gets scrambled). The reason to take hb->lock at this point is to avoid the wake_futex_pi() EAGAIN case. This happens when futex_top_waiter() and rt_mutex_top_waiter() state becomes inconsistent. The current rules are such that this inconsistency will not be observed. Notably the case that needs to be avoided is where futex_lock_pi() and futex_unlock_pi() interleave such that unlock will fail to observe a new waiter. *However* the case at hand is where a waiter is leaving, in this case the race means a waiter that is going away is not observed -- which is harmless, provided this race is explicitly handled. This is a somewhat dangerous proposition because the converse race is not observing a new waiter, which must absolutely not happen. But since the race is valid this cannot be asserted. Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Thomas Gleixner Reviewed-by: Sebastian Andrzej Siewior Tested-by: Sebastian Andrzej Siewior Link: https://lkml.kernel.org/r/20230915151943.GD6743@noisy.programming.kicks-ass.net --- kernel/futex/pi.c | 76 ++++++++++++++++++++++++++++++------------------- kernel/futex/requeue.c | 6 ++- 2 files changed, 52 insertions(+), 30 deletions(-) --- a/kernel/futex/pi.c +++ b/kernel/futex/pi.c @@ -611,29 +611,16 @@ int futex_lock_pi_atomic(u32 __user *uad /* * Caller must hold a reference on @pi_state. */ -static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state) +static int wake_futex_pi(u32 __user *uaddr, u32 uval, + struct futex_pi_state *pi_state, + struct rt_mutex_waiter *top_waiter) { - struct rt_mutex_waiter *top_waiter; struct task_struct *new_owner; bool postunlock = false; DEFINE_RT_WAKE_Q(wqh); u32 curval, newval; int ret = 0; - top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex); - if (WARN_ON_ONCE(!top_waiter)) { - /* - * As per the comment in futex_unlock_pi() this should not happen. - * - * When this happens, give up our locks and try again, giving - * the futex_lock_pi() instance time to complete, either by - * waiting on the rtmutex or removing itself from the futex - * queue. - */ - ret = -EAGAIN; - goto out_unlock; - } - new_owner = top_waiter->task; /* @@ -1046,20 +1033,34 @@ int futex_lock_pi(u32 __user *uaddr, uns ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter); cleanup: - spin_lock(q.lock_ptr); /* * If we failed to acquire the lock (deadlock/signal/timeout), we must - * first acquire the hb->lock before removing the lock from the - * rt_mutex waitqueue, such that we can keep the hb and rt_mutex wait - * lists consistent. + * must unwind the above, however we canont lock hb->lock because + * rt_mutex already has a waiter enqueued and hb->lock can itself try + * and enqueue an rt_waiter through rtlock. + * + * Doing the cleanup without holding hb->lock can cause inconsistent + * state between hb and pi_state, but only in the direction of not + * seeing a waiter that is leaving. + * + * See futex_unlock_pi(), it deals with this inconsistency. + * + * There be dragons here, since we must deal with the inconsistency on + * the way out (here), it is impossible to detect/warn about the race + * the other way around (missing an incoming waiter). * - * In particular; it is important that futex_unlock_pi() can not - * observe this inconsistency. + * What could possibly go wrong... */ if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter)) ret = 0; /* + * Now that the rt_waiter has been dequeued, it is safe to use + * spinlock/rtlock (which might enqueue its own rt_waiter) and fix up + * the + */ + spin_lock(q.lock_ptr); + /* * Waiter is unqueued. */ rt_mutex_post_schedule(); @@ -1143,6 +1144,7 @@ int futex_unlock_pi(u32 __user *uaddr, u top_waiter = futex_top_waiter(hb, &key); if (top_waiter) { struct futex_pi_state *pi_state = top_waiter->pi_state; + struct rt_mutex_waiter *rt_waiter; ret = -EINVAL; if (!pi_state) @@ -1155,22 +1157,39 @@ int futex_unlock_pi(u32 __user *uaddr, u if (pi_state->owner != current) goto out_unlock; - get_pi_state(pi_state); /* * By taking wait_lock while still holding hb->lock, we ensure - * there is no point where we hold neither; and therefore - * wake_futex_p() must observe a state consistent with what we - * observed. + * there is no point where we hold neither; and thereby + * wake_futex_pi() must observe any new waiters. + * + * Since the cleanup: case in futex_lock_pi() removes the + * rt_waiter without holding hb->lock, it is possible for + * wake_futex_pi() to not find a waiter while the above does, + * in this case the waiter is on the way out and it can be + * ignored. * * In particular; this forces __rt_mutex_start_proxy() to * complete such that we're guaranteed to observe the - * rt_waiter. Also see the WARN in wake_futex_pi(). + * rt_waiter. */ raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); + + /* + * Futex vs rt_mutex waiter state -- if there are no rt_mutex + * waiters even though futex thinks there are, then the waiter + * is leaving and the uncontended path is safe to take. + */ + rt_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex); + if (!rt_waiter) { + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); + goto do_uncontended; + } + + get_pi_state(pi_state); spin_unlock(&hb->lock); /* drops pi_state->pi_mutex.wait_lock */ - ret = wake_futex_pi(uaddr, uval, pi_state); + ret = wake_futex_pi(uaddr, uval, pi_state, rt_waiter); put_pi_state(pi_state); @@ -1198,6 +1217,7 @@ int futex_unlock_pi(u32 __user *uaddr, u return ret; } +do_uncontended: /* * We have no kernel internal state, i.e. no waiters in the * kernel. Waiters which are about to queue themselves are stuck --- a/kernel/futex/requeue.c +++ b/kernel/futex/requeue.c @@ -850,11 +850,13 @@ int futex_wait_requeue_pi(u32 __user *ua pi_mutex = &q.pi_state->pi_mutex; ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter); - /* Current is not longer pi_blocked_on */ - spin_lock(q.lock_ptr); + /* + * See futex_unlock_pi()'s cleanup: comment. + */ if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter)) ret = 0; + spin_lock(q.lock_ptr); debug_rt_mutex_free_waiter(&rt_waiter); /* * Fixup the pi_state owner and possibly acquire the lock if we