linux-kernel - Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read

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Message-ID: <20200918100432.GJ35926@hirez.programming.kicks-ass.net>
Date:   Fri, 18 Sep 2020 12:04:32 +0200
From:   peterz@...radead.org
To:     Jan Kara <jack@...e.cz>
Cc:     Oleg Nesterov <oleg@...hat.com>, Boaz Harrosh <boaz@...xistor.com>,
        Hou Tao <houtao1@...wei.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>, Dennis Zhou <dennis@...nel.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for
 read_count

On Fri, Sep 18, 2020 at 12:01:12PM +0200, peterz@...radead.org wrote:
> @@ -198,7 +198,9 @@ EXPORT_SYMBOL_GPL(__percpu_down_read);
>   */
>  static bool readers_active_check(struct percpu_rw_semaphore *sem)
>  {
> -	if (per_cpu_sum(*sem->read_count) != 0)
> +	u64 sum = per_cpu_sum(*(u64 *)sem->read_count);
> +
> +	if (sum + (sum >> 32))

That obviously wants to be:

	if ((u32)(sum + (sum >> 32)))

>  		return false;
>  
>  	/*