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Date:   Fri, 19 Apr 2019 15:15:22 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count
 negative
Message-ID: <20190419131522.GW14281@hirez.programming.kicks-ass.net>
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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.