linux-kernel - Re: [PATCH v11 06/16] qspinlock: prolong the stay in the pending bit path

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Date:	Sun, 15 Jun 2014 15:12:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v11 06/16] qspinlock: prolong the stay in the pending bit
 path

On Thu, Jun 12, 2014 at 04:54:52PM -0400, Waiman Long wrote:
> If two tasks see the pending bit goes away and try to grab it with cmpxchg,
> there is no way we can avoid the contention. However, if some how the
> pending bit holder get the lock and another task set the pending bit before
> the current task, the spinlock value will become
> _Q_PENDING_VAL|_Q_LOCKED_VAL. The while loop will end and the code will
> blindly try to do a cmpxchg unless we check for this case before hand. This
> is what my code does by going back to the beginning of the for loop.

There is already a test for that; see the goto queue;

---

	/*
	 * wait for in-progress pending->locked hand-overs
	 *
	 * 0,1,0 -> 0,0,1
	 */
	if (val == _Q_PENDING_VAL) {
		while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
			cpu_relax();
	}

	/*
	 * trylock || pending
	 *
	 * 0,0,0 -> 0,0,1 ; trylock
	 * 0,0,1 -> 0,1,1 ; pending
	 */
	for (;;) {
		/*
		 * If we observe any contention; queue.
		 */
		if (val & ~_Q_LOCKED_MASK)
			goto queue;

		new = _Q_LOCKED_VAL;
		if (val == new)
			new |= _Q_PENDING_VAL;

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

		val = old;
	}



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