linux-kernel - Re: [PATCH 2/4] lib/percpu-refcount: introduce percpu_ref

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Message-ID: <242969d0-1370-b342-025d-a11b7f59d28f@oracle.com>
Date:   Wed, 19 Sep 2018 13:19:10 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Ming Lei <ming.lei@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Tejun Heo <tj@...nel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-block@...r.kernel.org
Subject: Re: [PATCH 2/4] lib/percpu-refcount: introduce percpu_ref_resurge()

Hi Ming

On 09/18/2018 06:19 PM, Ming Lei wrote:
> +		unsigned long __percpu *percpu_count;
> +
> +		WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
> +
> +		/* get one extra ref for avoiding race with .release */
> +		rcu_read_lock_sched();
> +		atomic_long_add(1, &ref->count);
> +		rcu_read_unlock_sched();
> +	}

The rcu_read_lock_sched here is redundant. We have been in the critical section
of a spin_lock_irqsave.

The atomic_long_add(1, &ref->count) may have two result.
1. ref->count > 1
   it will not drop to zero any more.
2. ref->count == 1
   it has dropped to zero and .release may be running.

Thanks
Jianchao