linux-kernel - [tip: locking/core] futex: Sanitize exit state handling

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Message-ID: <157424270563.12247.11474112046933064148.tip-bot2@tip-bot2>
Date:   Wed, 20 Nov 2019 09:38:25 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Subject: [tip: locking/core] futex: Sanitize exit state handling

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

Commit-ID:     4a8e991b91aca9e20705d434677ac013974e0e30
Gitweb:        https://git.kernel.org/tip/4a8e991b91aca9e20705d434677ac013974e0e30
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Wed, 06 Nov 2019 22:55:42 +01:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Wed, 20 Nov 2019 09:40:09 +01:00

futex: Sanitize exit state handling

Instead of having a smp_mb() and an empty lock/unlock of task::pi_lock move
the state setting into to the lock section.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20191106224556.645603214@linutronix.de


---
 kernel/futex.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 3488fb0..f618562 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3703,16 +3703,19 @@ void futex_exit_recursive(struct task_struct *tsk)
 
 void futex_exit_release(struct task_struct *tsk)
 {
-	tsk->futex_state = FUTEX_STATE_EXITING;
-	/*
-	 * Ensure that all new tsk->pi_lock acquisitions must observe
-	 * FUTEX_STATE_EXITING. Serializes against attach_to_pi_owner().
-	 */
-	smp_mb();
 	/*
-	 * Ensure that we must observe the pi_state in exit_pi_state_list().
+	 * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
+	 *
+	 * This ensures that all subsequent checks of tsk->futex_state in
+	 * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
+	 * tsk->pi_lock held.
+	 *
+	 * It guarantees also that a pi_state which was queued right before
+	 * the state change under tsk->pi_lock by a concurrent waiter must
+	 * be observed in exit_pi_state_list().
 	 */
 	raw_spin_lock_irq(&tsk->pi_lock);
+	tsk->futex_state = FUTEX_STATE_EXITING;
 	raw_spin_unlock_irq(&tsk->pi_lock);
 
 	futex_exec_release(tsk);