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From d142001fdbdd8cc5fb558e9c3fa110f575875355 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 8 Mar 2017 14:23:35 +0100
Subject: [PATCH 060/347] futex: workaround migrate_disable/enable in different
context
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.246-rt110.tar.xz
migrate_disable()/migrate_enable() takes a different path in atomic() vs
!atomic() context. These little hacks ensure that we don't underflow / overflow
the migrate code counts properly while we lock the hb lockwith interrupts
enabled and unlock it with interrupts disabled.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/futex.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/kernel/futex.c b/kernel/futex.c
index 3c67da9b8408..fac994367189 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2998,6 +2998,14 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
* before __rt_mutex_start_proxy_lock() is done.
*/
raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
+ /*
+ * the migrate_disable() here disables migration in the in_atomic() fast
+ * path which is enabled again in the following spin_unlock(). We have
+ * one migrate_disable() pending in the slow-path which is reversed
+ * after the raw_spin_unlock_irq() where we leave the atomic context.
+ */
+ migrate_disable();
+
spin_unlock(q.lock_ptr);
/*
* __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
@@ -3006,6 +3014,7 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
*/
ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
+ migrate_enable();
if (ret) {
if (ret == 1)
@@ -3140,11 +3149,21 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
* rt_waiter. Also see the WARN in wake_futex_pi().
*/
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+ /*
+ * Magic trickery for now to make the RT migrate disable
+ * logic happy. The following spin_unlock() happens with
+ * interrupts disabled so the internal migrate_enable()
+ * won't undo the migrate_disable() which was issued when
+ * locking hb->lock.
+ */
+ migrate_disable();
spin_unlock(&hb->lock);
/* drops pi_state->pi_mutex.wait_lock */
ret = wake_futex_pi(uaddr, uval, pi_state);
+ migrate_enable();
+
put_pi_state(pi_state);
/*
--
2.36.1
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