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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        David Woodhouse <dwmw2@infradead.org>,
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        linux-csky@vger.kernel.org,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
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        "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
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        Mark Rutland <mark.rutland@arm.com>,
        Sabin Rapan <sabrapan@amazon.com>,
        "Michael Kelley (LINUX)" <mikelley@microsoft.com>
Subject: Re: [patch v3 18/36] [patch V2 18/38] cpu/hotplug: Add CPU state
 tracking and synchronization
In-Reply-To: <20230509110722.GZ83892@hirez.programming.kicks-ass.net>
References: <20230508181633.089804905@linutronix.de>
 <20230508185218.240871842@linutronix.de>
 <20230509110722.GZ83892@hirez.programming.kicks-ass.net>
Date:   Tue, 09 May 2023 14:12:12 +0200
Message-ID: <87bkit1z6r.ffs@tglx>
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On Tue, May 09 2023 at 13:07, Peter Zijlstra wrote:
> On Mon, May 08, 2023 at 09:43:55PM +0200, Thomas Gleixner wrote:
>
>> +static inline void cpuhp_ap_update_sync_state(enum cpuhp_sync_state state)
>> +{
>> +	atomic_t *st = this_cpu_ptr(&cpuhp_state.ap_sync_state);
>> +	int sync = atomic_read(st);
>> +
>> +	while (!atomic_try_cmpxchg(st, &sync, state));
>> +}
>
> Why isn't:
>
> 	atomic_set(st, state);
>
> any good?

Good question. It's only the other side (control CPU) which needs to be
careful.

Thanks,

        tglx