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

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Date:   Fri, 18 Sep 2020 12:48:24 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     peterz@...radead.org
Cc:     Jan Kara <jack@...e.cz>, 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 09/18, Peter Zijlstra wrote:
>
> On Fri, Sep 18, 2020 at 12:01:12PM +0200, peterz@...radead.org wrote:
> > +	u64 sum = per_cpu_sum(*(u64 *)sem->read_count);
>
> Moo, that doesn't work, we have to do two separate sums.

Or we can re-introduce "atomic_t slow_read_ctr".

	percpu_up_read_irqsafe(sem)
	{
		preempt_disable();
		atomic_dec_release(&sem->slow_read_ctr);
		if (!rcu_sync_is_idle(&sem->rss))
			rcuwait_wake_up(&sem->writer);
		preempt_enable();
	}

	readers_active_check(sem)
	{
		unsigned int sum = per_cpu_sum(*sem->read_count) +
			(unsigned int)atomic_read(&sem->slow_read_ctr);
		if (sum)
			return false;
		...
	}

Of course, this assumes that atomic_t->counter underflows "correctly", just
like "unsigned int".

But again, do we really want this?

Oleg.