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Message-ID: <9b149dd9-1617-9af4-4252-6d0df01f93b1@bytedance.com>
Date:   Tue, 25 Jul 2023 11:01:06 +0800
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Subject: Re: [PATCH v2 03/47] mm: shrinker: add infrastructure for dynamically
 allocating shrinker
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     akpm@linux-foundation.org, david@fromorbit.com, tkhai@ya.ru,
        vbabka@suse.cz, roman.gushchin@linux.dev, djwong@kernel.org,
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        yujie.liu@intel.com, gregkh@linuxfoundation.org,
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References: <20230724094354.90817-1-zhengqi.arch@bytedance.com>
 <20230724094354.90817-4-zhengqi.arch@bytedance.com>
 <20230724122549.GA3731903@hirez.programming.kicks-ass.net>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <20230724122549.GA3731903@hirez.programming.kicks-ass.net>
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Hi Peter,

On 2023/7/24 20:25, Peter Zijlstra wrote:
> On Mon, Jul 24, 2023 at 05:43:10PM +0800, Qi Zheng wrote:
> 
>> +void shrinker_unregister(struct shrinker *shrinker)
>> +{
>> +	struct dentry *debugfs_entry;
>> +	int debugfs_id;
>> +
>> +	if (!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))
>> +		return;
>> +
>> +	down_write(&shrinker_rwsem);
>> +	list_del(&shrinker->list);
>> +	shrinker->flags &= ~SHRINKER_REGISTERED;
>> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
>> +		unregister_memcg_shrinker(shrinker);
>> +	debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
>> +	up_write(&shrinker_rwsem);
>> +
>> +	shrinker_debugfs_remove(debugfs_entry, debugfs_id);
> 
> Should there not be an rcu_barrier() right about here?

The shrinker_debugfs_remove() will wait for debugfs_file_put() to
return, so when running here, all shrinker debugfs operations have
been completed. And the slab shrink will hold the read lock of
shrinker_rwsem to traverse the shrinker_list, so when we hold the
write lock of shrinker_rwsem to delete the shrinker from the
shrinker_list, the shrinker will not be executed again.

So I think there is no need to add a rcu_barrier() here. Please let
me know if I missed something.

Thanks,
Qi

> 
>> +
>> +	kfree(shrinker->nr_deferred);
>> +	shrinker->nr_deferred = NULL;
>> +
>> +	kfree(shrinker);
>> +}
>> +EXPORT_SYMBOL(shrinker_unregister);
>