linux-kernel - Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

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Date:   Mon, 28 Feb 2022 13:19:17 +0100
From:   Christian König <christian.koenig@....com>
To:     Jakob Koschel <jakobkoschel@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     alsa-devel@...a-project.org, linux-aspeed@...ts.ozlabs.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
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        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body
 as a ptr

Am 28.02.22 um 12:08 schrieb Jakob Koschel:
> If the list does not contain the expected element, the value of
> list_for_each_entry() iterator will not point to a valid structure.
> To avoid type confusion in such case, the list iterator
> scope will be limited to list_for_each_entry() loop.

We explicitly have the list_entry_is_head() macro to test after a loop 
if the element pointer points to the head of the list instead of a valid 
list entry.

So at least from my side I absolutely don't think that this is a good idea.

> In preparation to limiting scope of a list iterator to the list traversal
> loop, use a dedicated pointer to point to the found element.
> Determining if an element was found is then simply checking if
> the pointer is != NULL.

Since when do we actually want to do this?

Take this code here as an example:
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 48afe96ae0f0..6c916416decc 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -450,7 +450,8 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn,
>   				     struct mm_struct *mm)
>   {
>   	struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm, mmu_notifier);
> -	struct sgx_encl_mm *tmp = NULL;
> +	struct sgx_encl_mm *found_encl_mm = NULL;
> +	struct sgx_encl_mm *tmp;
>
>   	/*
>   	 * The enclave itself can remove encl_mm.  Note, objects can't be moved
> @@ -460,12 +461,13 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn,
>   	list_for_each_entry(tmp, &encl_mm->encl->mm_list, list) {
>   		if (tmp == encl_mm) {
>   			list_del_rcu(&encl_mm->list);
> +			found_encl_mm = tmp;
>   			break;
>   		}
>   	}
>   	spin_unlock(&encl_mm->encl->mm_lock);
>
> -	if (tmp == encl_mm) {
> +	if (found_encl_mm) {
>   		synchronize_srcu(&encl_mm->encl->srcu);
>   		mmu_notifier_put(mn);
>   	}

I don't think that using the extra variable makes the code in any way 
more reliable or easier to read.

Regards,
Christian.