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

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Date:   Fri, 18 Sep 2020 12:07:43 +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 12:04:32PM +0200, peterz@...radead.org wrote:
> 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;
> >  
> >  	/*

I suppose an alternative way of writing that would be something like:

	union {
		u64 sum;
		struct {
			u32 a, b;
		};
	} var;

	var.sum = per_cpu_sum(*(u64 *)sem->read_count);

	if (var.a + var.b)
		return false;

which is more verbose, but perhaps easier to read.