summaryrefslogtreecommitdiffstats
path: root/debian/patches-rt/0007-futex-pi-Fix-recursive-rt_mutex-waiter-state.patch
diff options
context:
space:
mode:
Diffstat (limited to 'debian/patches-rt/0007-futex-pi-Fix-recursive-rt_mutex-waiter-state.patch')
-rw-r--r--debian/patches-rt/0007-futex-pi-Fix-recursive-rt_mutex-waiter-state.patch198
1 files changed, 0 insertions, 198 deletions
diff --git a/debian/patches-rt/0007-futex-pi-Fix-recursive-rt_mutex-waiter-state.patch b/debian/patches-rt/0007-futex-pi-Fix-recursive-rt_mutex-waiter-state.patch
deleted file mode 100644
index 76a9b60581..0000000000
--- a/debian/patches-rt/0007-futex-pi-Fix-recursive-rt_mutex-waiter-state.patch
+++ /dev/null
@@ -1,198 +0,0 @@
-From: Peter Zijlstra <peterz@infradead.org>
-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) <peterz@infradead.org>
-Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
-Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
-Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
-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
generated by cgit v1.2.3 (git 2.39.1) at 2024-11-04 21:19:24 +0000