linux-kernel - Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath

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Message-ID: <20180920162216.GR24142@hirez.programming.kicks-ass.net>
Date:   Thu, 20 Sep 2018 18:22:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...nel.org,
        boqun.feng@...il.com, paulmck@...ux.vnet.ibm.com,
        catalin.marinas@....com
Subject: Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop
 from locking slowpath

On Thu, Sep 20, 2018 at 06:08:32PM +0200, Peter Zijlstra wrote:
> Another approach might be to use something like:
> 
> 	val = xchg_relaxed(&lock->locked_pending, _Q_PENDING_VAL | _Q_LOCKED_VAL);
> 	val |= atomic_read_acquire(&lock->val) & _Q_TAIL_MASK;
> 
> combined with something like:
> 
> 	/* 0,0,0 -> 0,1,1 - we won trylock */
> 	if (!(val & _Q_LOCKED_MASK)) {

That one doesn't actually work... let me think about this more.

> 		clear_pending(lock);
> 		return;
> 	}
> 
> 	/* 0,0,1 -> 0,1,1 - we won pending */
> 	if (!(val & ~_Q_LOCKED_MASK)) {
> 		...
> 	}
> 
> 	/* *,0,1 -> *,1,1 - we won pending, but there's queueing */
> 	if (!(val & _Q_PENDING_VAL))
> 		clear_pending(lock);
> 
> 	...
> 
> 
> Hmmm?