linux-kernel - [tip: locking/urgent] futex: Avoid redundant task lookup

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Message-ID: <163061250004.25758.18319268632920188158.tip-bot2@tip-bot2>
Date:   Thu, 02 Sep 2021 19:55:00 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/urgent] futex: Avoid redundant task lookup

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     c7e54b66384ca3e2c03e9021dc0627446dae2966
Gitweb:        https://git.kernel.org/tip/c7e54b66384ca3e2c03e9021dc0627446dae2966
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Thu, 02 Sep 2021 11:48:51 +02:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Thu, 02 Sep 2021 21:29:48 +02:00

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@...utronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20210902094414.676104881@linutronix.de

---
 kernel/futex.c | 67 +++++++++++++++++++++++++++----------------------
 1 file changed, 37 insertions(+), 30 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 82cd270..a316dce 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;
 	}