linux-kernel - Re: arc_usr_cmpxchg and preemption

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Message-ID: <20180315112829.GW4064@hirez.programming.kicks-ass.net>
Date:   Thu, 15 Mar 2018 12:28:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Cc:     "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: arc_usr_cmpxchg and preemption

On Thu, Mar 15, 2018 at 09:12:09AM +0000, Alexey Brodkin wrote:
> On Thu, 2018-03-15 at 09:18 +0100, Peter Zijlstra wrote:

> > Also, it might make sense to stuff this implementation in some lib/ file
> > somewhere and make all platforms that need it use the same code, afaict
> > there really isn't anything platform specific to it.
> 
> Not clear which part do you mean here.
> Are you talking about entire cmpxchg syscall implementation?

Yep.

> Do you think there're many users of that quite an inefficient
> [compared to proper HW version] atomic exchange?

I think there's a bunch of architectures that are in the same boat.
m68k, arm, mips was mentioned. Sure, the moment an arch has hardware
support you don't need the syscall anymore.

I was just thinking it would be good to have a common implementation (if
possible) rather than 4-5 different copies of basically the same thing.