Commit 9a4b99fc authored by Davidlohr Bueso's avatar Davidlohr Bueso Committed by Thomas Gleixner
Browse files

kernel/futex: Kill rt_mutex_next_owner() 



Update wake_futex_pi() and kill the call altogether. This is possible because:

(i) The case of fixup_owner() in which the pi_mutex was stolen from the
signaled enqueued top-waiter which fails to trylock and doesn't see a
current owner of the rtmutex but needs to acknowledge an non-enqueued
higher priority waiter, which is the other alternative. This used to be
handled by rt_mutex_next_owner(), which guaranteed fixup_pi_state_owner('newowner')
never to be nil. Nowadays the logic is handled by an EAGAIN loop, without
the need of rt_mutex_next_owner(). Specifically:

    c1e2f0ea (futex: Avoid violating the 10th rule of futex)
    9f5d1c33 (futex: Handle transient "ownerless" rtmutex state correctly)

(ii) rt_mutex_next_owner() and rt_mutex_top_waiter() are semantically
equivalent, as of:

    c28d62cf (locking/rtmutex: Handle non enqueued waiters gracefully in remove_waiter())

So instead of keeping the call around, just use the good ole rt_mutex_top_waiter().
No change in semantics.

Signed-off-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210226175029.50335-1-dave@stgolabs.net
parent bdb1050e

kernel/futex.c

+5 −2
Original line number Diff line number Diff line
@@ -1494,13 +1494,14 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q) 
static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)

{

	u32 curval, newval;

	struct rt_mutex_waiter *top_waiter;

	struct task_struct *new_owner;

	bool postunlock = false;

	DEFINE_WAKE_Q(wake_q);

	int ret = 0;



	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);

	if (WARN_ON_ONCE(!new_owner)) {

	top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);

	if (WARN_ON_ONCE(!top_waiter)) {

		/*

		 * As per the comment in futex_unlock_pi() this should not happen.

		 *
@@ -1513,6 +1514,8 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_ 
		goto out_unlock;

	}



	new_owner = top_waiter->task;



	/*

	 * We pass it to the next owner. The WAITERS bit is always kept

	 * enabled while there is PI state around. We cleanup the owner

kernel/locking/rtmutex.c

+0 −20
Original line number Diff line number Diff line
@@ -1792,26 +1792,6 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock, 
	return ret;

}



/**

 * rt_mutex_next_owner - return the next owner of the lock

 *

 * @lock: the rt lock query

 *

 * Returns the next owner of the lock or NULL

 *

 * Caller has to serialize against other accessors to the lock

 * itself.

 *

 * Special API call for PI-futex support

 */

struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock)

{

	if (!rt_mutex_has_waiters(lock))

		return NULL;



	return rt_mutex_top_waiter(lock)->task;

}



/**

 * rt_mutex_wait_proxy_lock() - Wait for lock acquisition

 * @lock:		the rt_mutex we were woken on

kernel/locking/rtmutex_common.h

+0 −1
Original line number Diff line number Diff line
@@ -130,7 +130,6 @@ enum rtmutex_chainwalk { 
/*

 * PI-futex support (proxy locking functions, etc.):

 */

extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);

extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,

				       struct task_struct *proxy_owner);

extern void rt_mutex_proxy_unlock(struct rt_mutex *lock);