linux-kernel - Re: [PATCH v2 1/7] proc/kcore: don't grab lock for kclist

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Date:   Tue, 17 Jul 2018 19:35:04 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Omar Sandoval <osandov@...ndov.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        James Morse <james.morse@....com>,
        Bhupesh Sharma <bhsharma@...hat.com>, kernel-team@...com
Subject: Re: [PATCH v2 1/7] proc/kcore: don't grab lock for kclist_add()

On Thu, 12 Jul 2018 17:09:33 -0700 Omar Sandoval <osandov@...ndov.com> wrote:

> From: Omar Sandoval <osandov@...com>
> 
> kclist_add() is only called at init time, so there's no point in
> grabbing any locks. We're also going to replace the rwlock with a rwsem,
> which we don't want to try grabbing during early boot.
> 
> ...
>
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -62,6 +62,7 @@ static LIST_HEAD(kclist_head);
>  static DEFINE_RWLOCK(kclist_lock);
>  static int kcore_need_update = 1;
>  
> +/* This doesn't grab kclist_lock, so it should only be used at init time. */
>  void
>  kclist_add(struct kcore_list *new, void *addr, size_t size, int type)
>  {
> @@ -69,9 +70,7 @@ kclist_add(struct kcore_list *new, void *addr, size_t size, int type)
>  	new->size = size;
>  	new->type = type;
>  
> -	write_lock(&kclist_lock);
>  	list_add_tail(&new->list, &kclist_head);
> -	write_unlock(&kclist_lock);
>  }

So we can mark kclist_add() as __init, yes?

That way we save a scrap of ram and if someone starts calling
kclist_add() from non-__init code we get a build-time warning.

--- a/fs/proc/kcore.c~proc-kcore-dont-grab-lock-for-kclist_add-fix
+++ a/fs/proc/kcore.c
@@ -63,7 +63,7 @@ static DEFINE_RWLOCK(kclist_lock);
 static int kcore_need_update = 1;
 
 /* This doesn't grab kclist_lock, so it should only be used at init time. */
-void
+void __init
 kclist_add(struct kcore_list *new, void *addr, size_t size, int type)
 {
 	new->addr = (unsigned long)addr;
--- a/include/linux/kcore.h~proc-kcore-dont-grab-lock-for-kclist_add-fix
+++ a/include/linux/kcore.h
@@ -35,7 +35,7 @@ struct vmcoredd_node {
 };
 
 #ifdef CONFIG_PROC_KCORE
-extern void kclist_add(struct kcore_list *, void *, size_t, int type);
+extern void __init kclist_add(struct kcore_list *, void *, size_t, int type);
 #else
 static inline
 void kclist_add(struct kcore_list *new, void *addr, size_t size, int type)