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Commit 6403bd7d authored by Waiman Long's avatar Waiman Long Committed by Ingo Molnar
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locking/qspinlock: Extract out code snippets for the next patch 



This is a preparatory patch that extracts out the following 2 code
snippets to prepare for the next performance optimization patch.

 1) the logic for the exchange of new and previous tail code words
    into a new xchg_tail() function.
 2) the logic for clearing the pending bit and setting the locked bit
    into a new clear_pending_set_locked() function.

This patch also simplifies the trylock operation before queuing by
calling queued_spin_trylock() directly.

Signed-off-by: default avatarWaiman Long <Waiman.Long@hp.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Daniel J Blueman <daniel@numascale.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Douglas Hatch <doug.hatch@hp.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paolo Bonzini <paolo.bonzini@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: virtualization@lists.linux-foundation.org
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/1429901803-29771-5-git-send-email-Waiman.Long@hp.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent c1fb159d

include/asm-generic/qspinlock_types.h

+2 −0
Original line number Diff line number Diff line
@@ -58,6 +58,8 @@ typedef struct qspinlock { 
#define _Q_TAIL_CPU_BITS	(32 - _Q_TAIL_CPU_OFFSET)

#define _Q_TAIL_CPU_MASK	_Q_SET_MASK(TAIL_CPU)



#define _Q_TAIL_MASK		(_Q_TAIL_IDX_MASK | _Q_TAIL_CPU_MASK)



#define _Q_LOCKED_VAL		(1U << _Q_LOCKED_OFFSET)

#define _Q_PENDING_VAL		(1U << _Q_PENDING_OFFSET)

kernel/locking/qspinlock.c

+48 −31
Original line number Diff line number Diff line
@@ -96,6 +96,42 @@ static inline struct mcs_spinlock *decode_tail(u32 tail) 


#define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)



/**

 * clear_pending_set_locked - take ownership and clear the pending bit.

 * @lock: Pointer to queued spinlock structure

 *

 * *,1,0 -> *,0,1

 */

static __always_inline void clear_pending_set_locked(struct qspinlock *lock)

{

	atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);

}



/**

 * xchg_tail - Put in the new queue tail code word & retrieve previous one

 * @lock : Pointer to queued spinlock structure

 * @tail : The new queue tail code word

 * Return: The previous queue tail code word

 *

 * xchg(lock, tail)

 *

 * p,*,* -> n,*,* ; prev = xchg(lock, node)

 */

static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)

{

	u32 old, new, val = atomic_read(&lock->val);



	for (;;) {

		new = (val & _Q_LOCKED_PENDING_MASK) | tail;

		old = atomic_cmpxchg(&lock->val, val, new);

		if (old == val)

			break;



		val = old;

	}

	return old;

}



/**

 * queued_spin_lock_slowpath - acquire the queued spinlock

 * @lock: Pointer to queued spinlock structure
@@ -178,15 +214,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) 
	 *

	 * *,1,0 -> *,0,1

	 */

	for (;;) {

		new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;



		old = atomic_cmpxchg(&lock->val, val, new);

		if (old == val)

			break;



		val = old;

	}

	clear_pending_set_locked(lock);

	return;



	/*
@@ -203,37 +231,26 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) 
	node->next = NULL;



	/*

	 * We have already touched the queueing cacheline; don't bother with

	 * pending stuff.

	 *

	 * trylock || xchg(lock, node)

	 *

	 * 0,0,0 -> 0,0,1 ; no tail, not locked -> no tail, locked.

	 * p,y,x -> n,y,x ; tail was p -> tail is n; preserving locked.

	 * We touched a (possibly) cold cacheline in the per-cpu queue node;

	 * attempt the trylock once more in the hope someone let go while we

	 * weren't watching.

	 */

	for (;;) {

		new = _Q_LOCKED_VAL;

		if (val)

			new = tail | (val & _Q_LOCKED_PENDING_MASK);



		old = atomic_cmpxchg(&lock->val, val, new);

		if (old == val)

			break;



		val = old;

	}

	if (queued_spin_trylock(lock))

		goto release;



	/*

	 * we won the trylock; forget about queueing.

	 * We have already touched the queueing cacheline; don't bother with

	 * pending stuff.

	 *

	 * p,*,* -> n,*,*

	 */

	if (new == _Q_LOCKED_VAL)

		goto release;

	old = xchg_tail(lock, tail);



	/*

	 * if there was a previous node; link it and wait until reaching the

	 * head of the waitqueue.

	 */

	if (old & ~_Q_LOCKED_PENDING_MASK) {

	if (old & _Q_TAIL_MASK) {

		prev = decode_tail(old);

		WRITE_ONCE(prev->next, node);