futex: Change locking rules
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>
This commit is contained in:
parent
5293c2efda
commit
734009e96d
163
kernel/futex.c
163
kernel/futex.c
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@ -973,6 +973,39 @@ void exit_pi_state_list(struct task_struct *curr)
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*
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* [10] There is no transient state which leaves owner and user space
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* TID out of sync.
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*
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*
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* Serialization and lifetime rules:
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*
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* hb->lock:
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*
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* hb -> futex_q, relation
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* futex_q -> pi_state, relation
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*
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* (cannot be raw because hb can contain arbitrary amount
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* of futex_q's)
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*
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* pi_mutex->wait_lock:
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*
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* {uval, pi_state}
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*
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* (and pi_mutex 'obviously')
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*
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* p->pi_lock:
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*
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* p->pi_state_list -> pi_state->list, relation
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*
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* pi_state->refcount:
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*
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* pi_state lifetime
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*
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*
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* Lock order:
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*
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* hb->lock
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* pi_mutex->wait_lock
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* p->pi_lock
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*
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*/
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/*
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@ -980,10 +1013,12 @@ void exit_pi_state_list(struct task_struct *curr)
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* the pi_state against the user space value. If correct, attach to
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* it.
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*/
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static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state,
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static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
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struct futex_pi_state *pi_state,
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struct futex_pi_state **ps)
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{
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pid_t pid = uval & FUTEX_TID_MASK;
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int ret, uval2;
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/*
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* Userspace might have messed up non-PI and PI futexes [3]
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@ -991,8 +1026,33 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state,
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if (unlikely(!pi_state))
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return -EINVAL;
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/*
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* We get here with hb->lock held, and having found a
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* futex_top_waiter(). This means that futex_lock_pi() of said futex_q
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* has dropped the hb->lock in between queue_me() and unqueue_me_pi(),
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* which in turn means that futex_lock_pi() still has a reference on
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* our pi_state.
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*/
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WARN_ON(!atomic_read(&pi_state->refcount));
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/*
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* Now that we have a pi_state, we can acquire wait_lock
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* and do the state validation.
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*/
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raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
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/*
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* Since {uval, pi_state} is serialized by wait_lock, and our current
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* uval was read without holding it, it can have changed. Verify it
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* still is what we expect it to be, otherwise retry the entire
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* operation.
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*/
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if (get_futex_value_locked(&uval2, uaddr))
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goto out_efault;
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if (uval != uval2)
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goto out_eagain;
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/*
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* Handle the owner died case:
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*/
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@ -1008,11 +1068,11 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state,
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* is not 0. Inconsistent state. [5]
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*/
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if (pid)
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return -EINVAL;
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goto out_einval;
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/*
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* Take a ref on the state and return success. [4]
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*/
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goto out_state;
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goto out_attach;
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}
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/*
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@ -1024,14 +1084,14 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state,
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* Take a ref on the state and return success. [6]
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*/
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if (!pid)
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goto out_state;
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goto out_attach;
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} else {
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/*
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* If the owner died bit is not set, then the pi_state
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* must have an owner. [7]
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*/
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if (!pi_state->owner)
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return -EINVAL;
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goto out_einval;
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}
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/*
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@ -1040,11 +1100,29 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state,
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* user space TID. [9/10]
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*/
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if (pid != task_pid_vnr(pi_state->owner))
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return -EINVAL;
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out_state:
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goto out_einval;
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out_attach:
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atomic_inc(&pi_state->refcount);
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raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
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*ps = pi_state;
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return 0;
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out_einval:
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ret = -EINVAL;
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goto out_error;
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out_eagain:
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ret = -EAGAIN;
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goto out_error;
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out_efault:
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ret = -EFAULT;
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goto out_error;
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out_error:
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raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
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return ret;
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}
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/*
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@ -1095,6 +1173,9 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
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/*
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* No existing pi state. First waiter. [2]
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*
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* This creates pi_state, we have hb->lock held, this means nothing can
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* observe this state, wait_lock is irrelevant.
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*/
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pi_state = alloc_pi_state();
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@ -1119,7 +1200,8 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
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return 0;
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}
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static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
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static int lookup_pi_state(u32 __user *uaddr, u32 uval,
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struct futex_hash_bucket *hb,
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union futex_key *key, struct futex_pi_state **ps)
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{
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struct futex_q *top_waiter = futex_top_waiter(hb, key);
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@ -1129,7 +1211,7 @@ static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
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* attach to the pi_state when the validation succeeds.
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*/
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if (top_waiter)
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return attach_to_pi_state(uval, top_waiter->pi_state, ps);
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return attach_to_pi_state(uaddr, uval, top_waiter->pi_state, ps);
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/*
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* We are the first waiter - try to look up the owner based on
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@ -1204,7 +1286,7 @@ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
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*/
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top_waiter = futex_top_waiter(hb, key);
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if (top_waiter)
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return attach_to_pi_state(uval, top_waiter->pi_state, ps);
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return attach_to_pi_state(uaddr, uval, top_waiter->pi_state, ps);
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/*
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* No waiter and user TID is 0. We are here because the
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@ -1336,6 +1418,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter
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if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) {
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ret = -EFAULT;
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} else if (curval != uval) {
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/*
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* If a unconditional UNLOCK_PI operation (user space did not
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@ -1348,6 +1431,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter
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else
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ret = -EINVAL;
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}
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if (ret) {
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raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
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return ret;
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@ -1823,7 +1907,7 @@ retry_private:
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* If that call succeeds then we have pi_state and an
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* initial refcount on it.
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*/
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ret = lookup_pi_state(ret, hb2, &key2, &pi_state);
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ret = lookup_pi_state(uaddr2, ret, hb2, &key2, &pi_state);
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}
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switch (ret) {
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@ -2122,10 +2206,13 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
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{
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u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
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struct futex_pi_state *pi_state = q->pi_state;
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struct task_struct *oldowner = pi_state->owner;
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u32 uval, uninitialized_var(curval), newval;
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struct task_struct *oldowner;
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int ret;
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raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
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oldowner = pi_state->owner;
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/* Owner died? */
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if (!pi_state->owner)
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newtid |= FUTEX_OWNER_DIED;
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@ -2141,11 +2228,10 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
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* because we can fault here. Imagine swapped out pages or a fork
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* that marked all the anonymous memory readonly for cow.
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*
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* Modifying pi_state _before_ the user space value would
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* leave the pi_state in an inconsistent state when we fault
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* here, because we need to drop the hash bucket lock to
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* handle the fault. This might be observed in the PID check
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* in lookup_pi_state.
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* Modifying pi_state _before_ the user space value would leave the
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* pi_state in an inconsistent state when we fault here, because we
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* need to drop the locks to handle the fault. This might be observed
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* in the PID check in lookup_pi_state.
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*/
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retry:
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if (get_futex_value_locked(&uval, uaddr))
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@ -2166,47 +2252,60 @@ retry:
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* itself.
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*/
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if (pi_state->owner != NULL) {
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raw_spin_lock_irq(&pi_state->owner->pi_lock);
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raw_spin_lock(&pi_state->owner->pi_lock);
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WARN_ON(list_empty(&pi_state->list));
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list_del_init(&pi_state->list);
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raw_spin_unlock_irq(&pi_state->owner->pi_lock);
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raw_spin_unlock(&pi_state->owner->pi_lock);
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}
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pi_state->owner = newowner;
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raw_spin_lock_irq(&newowner->pi_lock);
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raw_spin_lock(&newowner->pi_lock);
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WARN_ON(!list_empty(&pi_state->list));
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list_add(&pi_state->list, &newowner->pi_state_list);
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raw_spin_unlock_irq(&newowner->pi_lock);
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raw_spin_unlock(&newowner->pi_lock);
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raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
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return 0;
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/*
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* To handle the page fault we need to drop the hash bucket
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* lock here. That gives the other task (either the highest priority
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* waiter itself or the task which stole the rtmutex) the
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* chance to try the fixup of the pi_state. So once we are
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* back from handling the fault we need to check the pi_state
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* after reacquiring the hash bucket lock and before trying to
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* do another fixup. When the fixup has been done already we
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* simply return.
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* To handle the page fault we need to drop the locks here. That gives
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* the other task (either the highest priority waiter itself or the
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* task which stole the rtmutex) the chance to try the fixup of the
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* pi_state. So once we are back from handling the fault we need to
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* check the pi_state after reacquiring the locks and before trying to
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* do another fixup. When the fixup has been done already we simply
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* return.
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*
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* Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely
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* drop hb->lock since the caller owns the hb -> futex_q relation.
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* Dropping the pi_mutex->wait_lock requires the state revalidate.
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*/
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handle_fault:
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raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
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spin_unlock(q->lock_ptr);
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ret = fault_in_user_writeable(uaddr);
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spin_lock(q->lock_ptr);
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raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
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/*
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* Check if someone else fixed it for us:
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*/
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if (pi_state->owner != oldowner)
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return 0;
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if (pi_state->owner != oldowner) {
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ret = 0;
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goto out_unlock;
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}
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if (ret)
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return ret;
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goto out_unlock;
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goto retry;
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out_unlock:
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raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
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return ret;
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}
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static long futex_wait_restart(struct restart_block *restart);
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