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Date:   Fri, 19 Apr 2019 15:15:22 +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 14/16] locking/rwsem: Guard against making count
 negative

On Fri, Apr 19, 2019 at 03:03:04PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 19, 2019 at 02:02:07PM +0200, Peter Zijlstra wrote:
> > On Fri, Apr 19, 2019 at 12:26:47PM +0200, Peter Zijlstra wrote:
> > > I thought of a horrible horrible alternative:
> > 
> > Hurm, that's broken as heck. Let me try again.
> 
> So I can't make that scheme work, it all ends up wanting to have
> cmpxchg().
> 
> Do we have a performance comparison somewhere of xadd vs cmpxchg
> readers? I tried looking in the old threads, but I can't seem to locate
> it.
> 
> We need new instructions :/ Or more clever than I can muster just now.

In particular, an (unsigned) saturation arithmetic variant of XADD would
be very nice to have at this point.

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