From: Peter Zijlstra peterz@infradead.org
Currently futex-pi relies on hb->lock to serialize everything. But hb->lock creates another set of problems, especially priority inversions on RT where hb->lock becomes a rt_mutex itself.
The rt_mutex::wait_lock is the most obvious protection for keeping the futex user space value and the kernel internal pi_state in sync.
Rework and document the locking so rt_mutex::wait_lock is held accross all operations which modify the user space value and the pi state.
This allows to invoke rt_mutex_unlock() (including deboost) without holding hb->lock as a next step.
Nothing yet relies on the new locking rules.
Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: juri.lelli@arm.com Cc: bigeasy@linutronix.de 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/20170322104151.751993333@infradead.org Signed-off-by: Thomas Gleixner tglx@linutronix.de [Lee: Back-ported in support of a previous futex back-port attempt] Signed-off-by: Lee Jones lee.jones@linaro.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/futex.c | 138 ++++++++++++++++++++++++++++++++++++++++++++++----------- 1 file changed, 112 insertions(+), 26 deletions(-)
--- a/kernel/futex.c +++ b/kernel/futex.c @@ -1019,6 +1019,39 @@ static void exit_pi_state_list(struct ta * [10] There is no transient state which leaves owner and user space * TID out of sync. Except one error case where the kernel is denied * write access to the user address, see fixup_pi_state_owner(). + * + * + * Serialization and lifetime rules: + * + * hb->lock: + * + * hb -> futex_q, relation + * futex_q -> pi_state, relation + * + * (cannot be raw because hb can contain arbitrary amount + * of futex_q's) + * + * pi_mutex->wait_lock: + * + * {uval, pi_state} + * + * (and pi_mutex 'obviously') + * + * p->pi_lock: + * + * p->pi_state_list -> pi_state->list, relation + * + * pi_state->refcount: + * + * pi_state lifetime + * + * + * Lock order: + * + * hb->lock + * pi_mutex->wait_lock + * p->pi_lock + * */
/* @@ -1026,10 +1059,12 @@ static void exit_pi_state_list(struct ta * the pi_state against the user space value. If correct, attach to * it. */ -static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, +static int attach_to_pi_state(u32 __user *uaddr, u32 uval, + struct futex_pi_state *pi_state, struct futex_pi_state **ps) { pid_t pid = uval & FUTEX_TID_MASK; + int ret, uval2;
/* * Userspace might have messed up non-PI and PI futexes [3] @@ -1037,9 +1072,34 @@ static int attach_to_pi_state(u32 uval, if (unlikely(!pi_state)) return -EINVAL;
+ /* + * We get here with hb->lock held, and having found a + * futex_top_waiter(). This means that futex_lock_pi() of said futex_q + * has dropped the hb->lock in between queue_me() and unqueue_me_pi(), + * which in turn means that futex_lock_pi() still has a reference on + * our pi_state. + */ WARN_ON(!atomic_read(&pi_state->refcount));
/* + * Now that we have a pi_state, we can acquire wait_lock + * and do the state validation. + */ + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); + + /* + * Since {uval, pi_state} is serialized by wait_lock, and our current + * uval was read without holding it, it can have changed. Verify it + * still is what we expect it to be, otherwise retry the entire + * operation. + */ + if (get_futex_value_locked(&uval2, uaddr)) + goto out_efault; + + if (uval != uval2) + goto out_eagain; + + /* * Handle the owner died case: */ if (uval & FUTEX_OWNER_DIED) { @@ -1054,11 +1114,11 @@ static int attach_to_pi_state(u32 uval, * is not 0. Inconsistent state. [5] */ if (pid) - return -EINVAL; + goto out_einval; /* * Take a ref on the state and return success. [4] */ - goto out_state; + goto out_attach; }
/* @@ -1070,14 +1130,14 @@ static int attach_to_pi_state(u32 uval, * Take a ref on the state and return success. [6] */ if (!pid) - goto out_state; + goto out_attach; } else { /* * If the owner died bit is not set, then the pi_state * must have an owner. [7] */ if (!pi_state->owner) - return -EINVAL; + goto out_einval; }
/* @@ -1086,11 +1146,29 @@ static int attach_to_pi_state(u32 uval, * user space TID. [9/10] */ if (pid != task_pid_vnr(pi_state->owner)) - return -EINVAL; -out_state: + goto out_einval; + +out_attach: atomic_inc(&pi_state->refcount); + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); *ps = pi_state; return 0; + +out_einval: + ret = -EINVAL; + goto out_error; + +out_eagain: + ret = -EAGAIN; + goto out_error; + +out_efault: + ret = -EFAULT; + goto out_error; + +out_error: + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); + return ret; }
/** @@ -1183,6 +1261,9 @@ static int attach_to_pi_owner(u32 uval,
/* * No existing pi state. First waiter. [2] + * + * This creates pi_state, we have hb->lock held, this means nothing can + * observe this state, wait_lock is irrelevant. */ pi_state = alloc_pi_state();
@@ -1207,7 +1288,8 @@ static int attach_to_pi_owner(u32 uval, return 0; }
-static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, +static int lookup_pi_state(u32 __user *uaddr, u32 uval, + struct futex_hash_bucket *hb, union futex_key *key, struct futex_pi_state **ps, struct task_struct **exiting) { @@ -1218,7 +1300,7 @@ static int lookup_pi_state(u32 uval, str * attach to the pi_state when the validation succeeds. */ if (match) - return attach_to_pi_state(uval, match->pi_state, ps); + return attach_to_pi_state(uaddr, uval, match->pi_state, ps);
/* * We are the first waiter - try to look up the owner based on @@ -1237,7 +1319,7 @@ static int lock_pi_update_atomic(u32 __u if (unlikely(cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))) return -EFAULT;
- /*If user space value changed, let the caller retry */ + /* If user space value changed, let the caller retry */ return curval != uval ? -EAGAIN : 0; }
@@ -1301,7 +1383,7 @@ static int futex_lock_pi_atomic(u32 __us */ match = futex_top_waiter(hb, key); if (match) - return attach_to_pi_state(uval, match->pi_state, ps); + return attach_to_pi_state(uaddr, uval, match->pi_state, ps);
/* * No waiter and user TID is 0. We are here because the @@ -1441,6 +1523,7 @@ static int wake_futex_pi(u32 __user *uad
if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) { ret = -EFAULT; + } else if (curval != uval) { /* * If a unconditional UNLOCK_PI operation (user space did not @@ -1977,7 +2060,7 @@ retry_private: * If that call succeeds then we have pi_state and an * initial refcount on it. */ - ret = lookup_pi_state(ret, hb2, &key2, + ret = lookup_pi_state(uaddr2, ret, hb2, &key2, &pi_state, &exiting); }
@@ -2282,7 +2365,6 @@ static int __fixup_pi_state_owner(u32 __ int err = 0;
oldowner = pi_state->owner; - /* Owner died? */ if (!pi_state->owner) newtid |= FUTEX_OWNER_DIED; @@ -2305,11 +2387,10 @@ static int __fixup_pi_state_owner(u32 __ * because we can fault here. Imagine swapped out pages or a fork * that marked all the anonymous memory readonly for cow. * - * Modifying pi_state _before_ the user space value would - * leave the pi_state in an inconsistent state when we fault - * here, because we need to drop the hash bucket lock to - * handle the fault. This might be observed in the PID check - * in lookup_pi_state. + * Modifying pi_state _before_ the user space value would leave the + * pi_state in an inconsistent state when we fault here, because we + * need to drop the locks to handle the fault. This might be observed + * in the PID check in lookup_pi_state. */ retry: if (!argowner) { @@ -2367,21 +2448,26 @@ retry: return argowner == current;
/* - * To handle the page fault we need to drop the hash bucket - * lock here. That gives the other task (either the highest priority - * waiter itself or the task which stole the rtmutex) the - * chance to try the fixup of the pi_state. So once we are - * back from handling the fault we need to check the pi_state - * after reacquiring the hash bucket lock and before trying to - * do another fixup. When the fixup has been done already we - * simply return. + * To handle the page fault we need to drop the locks here. That gives + * the other task (either the highest priority waiter itself or the + * task which stole the rtmutex) the chance to try the fixup of the + * pi_state. So once we are back from handling the fault we need to + * check the pi_state after reacquiring the locks and before trying to + * do another fixup. When the fixup has been done already we simply + * return. + * + * Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely + * drop hb->lock since the caller owns the hb -> futex_q relation. + * Dropping the pi_mutex->wait_lock requires the state revalidate. */ handle_fault: + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); spin_unlock(q->lock_ptr);
err = fault_in_user_writeable(uaddr);
spin_lock(q->lock_ptr); + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
/* * Check if someone else fixed it for us: