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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.

  reply	other threads:[~2020-09-18 10:08 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-15 14:07 Hou Tao
2020-09-15 15:06 ` peterz
2020-09-15 15:31   ` Oleg Nesterov
2020-09-15 15:51     ` peterz
2020-09-15 16:03       ` peterz
2020-09-15 16:11         ` Will Deacon
2020-09-15 18:11           ` peterz
2020-09-16  8:20             ` Will Deacon
2020-09-15 16:47         ` Oleg Nesterov
2020-09-16 12:32         ` Hou Tao
2020-09-16 12:51           ` peterz
2020-09-17  8:48           ` Will Deacon
2020-09-24 11:55             ` Hou Tao
2020-09-29 17:49               ` Will Deacon
2020-09-29 18:07                 ` Ard Biesheuvel
2020-09-17 10:51           ` Boaz Harrosh
2020-09-17 12:01             ` Oleg Nesterov
2020-09-17 12:48               ` Matthew Wilcox
2020-09-17 13:22                 ` peterz
2020-09-17 13:34                 ` Oleg Nesterov
2020-09-17 13:46                 ` Boaz Harrosh
2020-09-17 14:46                   ` Christoph Hellwig
2020-09-18  9:07               ` Jan Kara
2020-09-18 10:01                 ` peterz
2020-09-18 10:04                   ` peterz
2020-09-18 10:07                     ` peterz [this message]
2020-09-18 10:12                   ` peterz
2020-09-18 10:48                     ` Oleg Nesterov
2020-09-18 11:03                       ` peterz
2020-09-18 13:09                         ` Oleg Nesterov
2020-09-18 13:26                           ` Jan Kara
2020-09-20 23:49                             ` Dave Chinner

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