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Date: Mon, 6 Jun 2016 16:46:59 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org,
 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
 Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Paul Mackerras <paulus@samba.org>,
 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
 Will Deacon <will.deacon@arm.com>, Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH v3] powerpc: spinlock: Fix spin_unlock_wait()
Message-ID: <20160606144659.GG30909@twins.programming.kicks-ass.net>
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On Mon, Jun 06, 2016 at 10:17:25PM +1000, Michael Ellerman wrote:
> On Mon, 2016-06-06 at 13:56 +0200, Peter Zijlstra wrote:
> > On Mon, Jun 06, 2016 at 09:42:20PM +1000, Michael Ellerman wrote:
> > > +static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
> > > +{
> > > +	arch_spinlock_t lock_val;
> > > +
> > > +	smp_mb();
> > > +
> > > +	/*
> > > +	 * Atomically load and store back the lock value (unchanged). This
> > > +	 * ensures that our observation of the lock value is ordered with
> > > +	 * respect to other lock operations.
> > > +	 */
> > > +	__asm__ __volatile__(
> > > +"1:	" PPC_LWARX(%0, 0, %2, 0) "\n"
> > > +"	stwcx. %0, 0, %2\n"
> > > +"	bne- 1b\n"
> > > +	: "=&r" (lock_val), "+m" (*lock)
> > > +	: "r" (lock)
> > > +	: "cr0", "xer");
> > > +
> > > +	if (arch_spin_value_unlocked(lock_val))
> > > +		goto out;
> > > +
> > > +	while (!arch_spin_value_unlocked(*lock)) {
> > > +		HMT_low();
> > > +		if (SHARED_PROCESSOR)
> > > +			__spin_yield(lock);
> > > +	}
> > > +	HMT_medium();
> > > +
> > > +out:
> > > +	smp_mb();
> > > +}
> > 
> > Why the move to in-line this implementation? It looks like a fairly big
> > function.
> 
> I agree it's not pretty.

> 
> I'm not beholden to v3 though if you hate it.

I don't mind; its just that I am in a similar boat with qspinlock and
chose the other option. So I just figured I'd ask :-)