linux-ext4 - Re: [PATCH v2 03/47] mm: shrinker: add infrastructure for dynamically allocating shrinker

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Message-ID: <9b149dd9-1617-9af4-4252-6d0df01f93b1@bytedance.com>
Date:   Tue, 25 Jul 2023 11:01:06 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     akpm@...ux-foundation.org, david@...morbit.com, tkhai@...ru,
        vbabka@...e.cz, roman.gushchin@...ux.dev, djwong@...nel.org,
        brauner@...nel.org, paulmck@...nel.org, tytso@....edu,
        steven.price@....com, cel@...nel.org, senozhatsky@...omium.org,
        yujie.liu@...el.com, gregkh@...uxfoundation.org,
        muchun.song@...ux.dev, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-erofs@...ts.ozlabs.org,
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Subject: Re: [PATCH v2 03/47] mm: shrinker: add infrastructure for dynamically
 allocating shrinker

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);
>