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Date:   Wed, 17 Apr 2019 15:41:52 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mateusz Guzik <mguzik@redhat.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Arun KS <arunks@codeaurora.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: get_cmdline use arg_lock instead of mmap_sem
Message-ID: <20190417134152.GM5878@dhcp22.suse.cz>
References: <20190417120347.15397-1-mkoutny@suse.com>
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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 Koutn? <mkoutny@suse.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