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

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Date:   Thu, 17 Feb 2022 13:32:24 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
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 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.

>  }
>  
>  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.