linux-kernel - Re: [PATCH] mm: get_cmdline use arg_lock instead of mmap

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Date:   Wed, 17 Apr 2019 15:41:52 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mateusz Guzik <mguzik@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Arun KS <arunks@...eaurora.org>,
        Bartosz Golaszewski <brgl@...ev.pl>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: get_cmdline use arg_lock instead of mmap_sem

On Wed 17-04-19 14:03:47, Michal Koutny wrote:
> The commit a3b609ef9f8b ("proc read mm's {arg,env}_{start,end} with mmap
> semaphore taken.") added synchronization of reading argument/environment
> boundaries under mmap_sem. Later commit 88aa7cc688d4 ("mm: introduce
> arg_lock to protect arg_start|end and env_start|end in mm_struct")
> avoided the coarse use of mmap_sem in similar situations.
> 
> get_cmdline can also use arg_lock instead of mmap_sem when it reads the
> boundaries.

Don't we need to use the lock in prctl_set_mm as well then?

> Signed-off-by: Michal Koutný <mkoutny@...e.com>
> ---
>  mm/util.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index d559bde497a9..568575cceefc 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -758,12 +758,12 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
>  	if (!mm->arg_end)
>  		goto out_mm;	/* Shh! No looking before we're done */
>  
> -	down_read(&mm->mmap_sem);
> +	spin_lock(&mm->arg_lock);
>  	arg_start = mm->arg_start;
>  	arg_end = mm->arg_end;
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
>  
>  	len = arg_end - arg_start;
>  
> -- 
> 2.16.4

-- 
Michal Hocko
SUSE Labs