1 files changed, 122 insertions, 0 deletions
diff --git a/patches/0004-futex-Avoid-redundant-task-lookup.patch b/patches/0004-futex-Avoid-redundant-task-lookup.patch
new file mode 100644
index 000000000000..ae99fce2404d
--- /dev/null
+++ b/patches/0004-futex-Avoid-redundant-task-lookup.patch
@@ -0,0 +1,122 @@
+From: Thomas Gleixner <tglx@linutronix.de>
+Date: Thu, 2 Sep 2021 11:48:51 +0200
+Subject: [PATCH 4/4] futex: Avoid redundant task lookup
+
+No need to do the full VPID based task lookup and validation of the top
+waiter when the user space futex was acquired on it's behalf during the
+requeue_pi operation. The task is known already and it cannot go away
+before requeue_pi_wake_futex() has been invoked.
+
+Split out the actual attach code from attach_pi_state_owner() and use that
+instead of the full blown variant.
+
+Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
+Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
+Link: https://lore.kernel.org/r/20210902094414.676104881@linutronix.de
+---
+ kernel/futex.c |   67 +++++++++++++++++++++++++++++++--------------------------
+ 1 file changed, 37 insertions(+), 30 deletions(-)
+
+--- a/kernel/futex.c
++++ b/kernel/futex.c
+@@ -1263,6 +1263,36 @@ static int handle_exit_race(u32 __user *
+ 	return -ESRCH;
+ }
+ 
++static void __attach_to_pi_owner(struct task_struct *p, union futex_key *key,
++				 struct futex_pi_state **ps)
++{
++	/*
++	 * 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.
++	 */
++	struct futex_pi_state *pi_state = alloc_pi_state();
++
++	/*
++	 * Initialize the pi_mutex in locked state and make @p
++	 * the owner of it:
++	 */
++	rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);
++
++	/* Store the key for possible exit cleanups: */
++	pi_state->key = *key;
++
++	WARN_ON(!list_empty(&pi_state->list));
++	list_add(&pi_state->list, &p->pi_state_list);
++	/*
++	 * Assignment without holding pi_state->pi_mutex.wait_lock is safe
++	 * because there is no concurrency as the object is not published yet.
++	 */
++	pi_state->owner = p;
++
++	*ps = pi_state;
++}
+ /*
+  * Lookup the task for the TID provided from user space and attach to
+  * it after doing proper sanity checks.
+@@ -1272,7 +1302,6 @@ static int attach_to_pi_owner(u32 __user
+ 			      struct task_struct **exiting)
+ {
+ 	pid_t pid = uval & FUTEX_TID_MASK;
+-	struct futex_pi_state *pi_state;
+ 	struct task_struct *p;
+ 
+ 	/*
+@@ -1324,36 +1353,11 @@ static int attach_to_pi_owner(u32 __user
+ 		return ret;
+ 	}
+ 
+-	/*
+-	 * 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();
+-
+-	/*
+-	 * Initialize the pi_mutex in locked state and make @p
+-	 * the owner of it:
+-	 */
+-	rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);
+-
+-	/* Store the key for possible exit cleanups: */
+-	pi_state->key = *key;
+-
+-	WARN_ON(!list_empty(&pi_state->list));
+-	list_add(&pi_state->list, &p->pi_state_list);
+-	/*
+-	 * Assignment without holding pi_state->pi_mutex.wait_lock is safe
+-	 * because there is no concurrency as the object is not published yet.
+-	 */
+-	pi_state->owner = p;
++	__attach_to_pi_owner(p, key, ps);
+ 	raw_spin_unlock_irq(&p->pi_lock);
+ 
+ 	put_task_struct(p);
+ 
+-	*ps = pi_state;
+-
+ 	return 0;
+ }
+ 
+@@ -1464,11 +1468,14 @@ static int futex_lock_pi_atomic(u32 __us
+ 		 * @task is guaranteed to be alive and it cannot be exiting
+ 		 * because it is either sleeping or waiting in
+ 		 * futex_requeue_pi_wakeup_sync().
++		 *
++		 * No need to do the full attach_to_pi_owner() exercise
++		 * because @task is known and valid.
+ 		 */
+ 		if (set_waiters) {
+-			 ret = attach_to_pi_owner(uaddr, newval, key, ps,
+-						  exiting);
+-			 WARN_ON(ret);
++			raw_spin_lock_irq(&task->pi_lock);
++			__attach_to_pi_owner(task, key, ps);
++			raw_spin_unlock_irq(&task->pi_lock);
+ 		}
+ 		return 1;
+ 	}