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Diffstat (limited to 'kernel/futex/pi.c')
-rw-r--r--kernel/futex/pi.c91
1 files changed, 61 insertions, 30 deletions
diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
index ce2889f12375..90e5197f4e56 100644
--- a/kernel/futex/pi.c
+++ b/kernel/futex/pi.c
@@ -1,6 +1,7 @@
// SPDX-License-Identifier: GPL-2.0-or-later
#include <linux/slab.h>
+#include <linux/sched/rt.h>
#include <linux/sched/task.h>
#include "futex.h"
@@ -610,29 +611,16 @@ int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
/*
* 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;
/*
@@ -945,7 +933,7 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
to = futex_setup_timer(time, &timeout, flags, 0);
retry:
- ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, FUTEX_WRITE);
+ ret = get_futex_key(uaddr, flags, &q.key, FUTEX_WRITE);
if (unlikely(ret != 0))
goto out;
@@ -1002,6 +990,12 @@ retry_private:
goto no_block;
}
+ /*
+ * Must be done before we enqueue the waiter, here is unfortunately
+ * under the hb lock, but that *should* work because it does nothing.
+ */
+ rt_mutex_pre_schedule();
+
rt_mutex_init_waiter(&rt_waiter);
/*
@@ -1039,19 +1033,37 @@ retry_private:
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();
no_block:
/*
* Fixup the pi_state owner and possibly acquire the lock if we
@@ -1117,7 +1129,7 @@ retry:
if ((uval & FUTEX_TID_MASK) != vpid)
return -EPERM;
- ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, FUTEX_WRITE);
+ ret = get_futex_key(uaddr, flags, &key, FUTEX_WRITE);
if (ret)
return ret;
@@ -1132,6 +1144,7 @@ retry:
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)
@@ -1144,22 +1157,39 @@ retry:
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);
@@ -1187,6 +1217,7 @@ retry:
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