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

From: peterz@infradead.org
To: Jan Kara <jack@suse.cz>
Cc: Oleg Nesterov <oleg@redhat.com>,
	Boaz Harrosh <boaz@plexistor.com>, Hou Tao <houtao1@huawei.com>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
	Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
	Christoph Lameter <cl@linux.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count
Date: Fri, 18 Sep 2020 12:07:43 +0200	[thread overview]
Message-ID: <20200918100743.GK35926@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200918100432.GJ35926@hirez.programming.kicks-ass.net>

On Fri, Sep 18, 2020 at 12:04:32PM +0200, peterz@infradead.org wrote:
> On Fri, Sep 18, 2020 at 12:01:12PM +0200, peterz@infradead.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.