linux-kernel - Re: [PATCH v4 08/16] locking/rwsem: Make rwsem_spin_on

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Date:   Thu, 18 Apr 2019 10:39:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 08/16] locking/rwsem: Make rwsem_spin_on_owner()
 return owner state

On Wed, Apr 17, 2019 at 02:29:02PM -0400, Waiman Long wrote:
> On 04/17/2019 08:47 AM, Peter Zijlstra wrote:
> >>
> >> enum owner_state {
> >> 	OWNER_NULL		= 1 << 0,
> >> 	OWNER_WRITER		= 1 << 1,
> >> 	OWNER_READER		= 1 << 2,
> >> 	OWNER_NONSPINNABLE	= 1 << 3,
> >> };
> >> #define OWNER_SPINNABLE		(OWNER_NULL | OWNER_WRITER)
> > Hmm, we should not spin on OWNER_NULL. Or at least not mixed in with the
> > patch that changes the shape of all this. That should go in the RT
> > thingy patch, which comes after this.
> 
> We do spin on OWNER_NULL right now, not in rwsem_spin_on_owner() but in
> the main rwsem_optimistic_spin() function.
> 
> RT task will quit if owner is NULL.

Yeah, I figured it out eventually :-)