From 340576590dac4bb58d532a8ad5bfa806d8ab473c Mon Sep 17 00:00:00 2001

From: Thomas Gleixner <tglx@linutronix.de>

Date: Thu, 2 Sep 2021 11:48:51 +0200

Subject: [PATCH] futex: Avoid redundant task lookup

Git-commit: 340576590dac4bb58d532a8ad5bfa806d8ab473c

Patch-mainline: v5.15-rc1

References: bsc#1190137 bsc#1189998

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

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

---

 kernel/futex.c | 67 ++++++++++++++++++++++++++++----------------------

 1 file changed, 37 insertions(+), 30 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c

index 82cd270f25a0..a316dce74c3d 100644

--- a/kernel/futex.c

+++ b/kernel/futex.c

@@ -1263,6 +1263,36 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval,

 	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 *uaddr, u32 uval, union futex_key *key,

 			      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 *uaddr, u32 uval, union futex_key *key,

 		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 __user *uaddr, struct futex_hash_bucket *hb,

 		 * @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;

 	}

-- 

2.26.2