linux-kernel - Re: [PATCH] mm/mmu_notifiers: use helper function mmu_notifier

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Date:   Thu, 17 Feb 2022 21:44:13 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/mmu_notifiers: use helper function
 mmu_notifier_synchronize()

On 2022/2/17 21:32, Matthew Wilcox wrote:
> On Thu, Feb 17, 2022 at 07:09:48PM +0800, Miaohe Lin wrote:
>> Use helper function mmu_notifier_synchronize() to ensure all mmu_notifiers
>> are freed. Minor readability improvement.
> 
> Is it though?
> 
>> @@ -334,15 +334,15 @@ static void mn_hlist_release(struct mmu_notifier_subscriptions *subscriptions,
>>  	srcu_read_unlock(&srcu, id);
>>  
>>  	/*
>> -	 * synchronize_srcu here prevents mmu_notifier_release from returning to
>> -	 * exit_mmap (which would proceed with freeing all pages in the mm)
>> -	 * until the ->release method returns, if it was invoked by
>> -	 * mmu_notifier_unregister.
>> +	 * mmu_notifier_synchronize here prevents mmu_notifier_release from
>> +	 * returning to exit_mmap (which would proceed with freeing all pages
>> +	 * in the mm) until the ->release method returns, if it was invoked
>> +	 * by mmu_notifier_unregister.
>>  	 *
>>  	 * The notifier_subscriptions can't go away from under us because
>>  	 * one mm_count is held by exit_mmap.
>>  	 */
>> -	synchronize_srcu(&srcu);
>> +	mmu_notifier_synchronize();
> 
> We just read_unlocked the &srcu.  Now I have to jump to the definition
> of mmu_notifier_synchronize() to find out that it's now waiting for the
> very same srcu.  I think this abstraction makes the code harder to read,
> not easier.
> 

>From this point of view, this helper would disturb the understanding of the code.
Many thanks for pointing this out. Sorry for my mindlessness.

>>  }
>>  
>>  void __mmu_notifier_release(struct mm_struct *mm)
>> @@ -851,7 +851,7 @@ void mmu_notifier_unregister(struct mmu_notifier *subscription,
>>  	 * Wait for any running method to finish, of course including
>>  	 * ->release if it was run by mmu_notifier_release instead of us.
>>  	 */
>> -	synchronize_srcu(&srcu);
>> +	mmu_notifier_synchronize();
> 
> Same here.
> 
> .
>