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Subject: Re: [PATCH 1/3] mm: get_cmdline use arg_lock instead of mmap_sem
To:     =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        gorcunov@gmail.com
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        geert+renesas@glider.be, ldufour@linux.ibm.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        mguzik@redhat.com, mhocko@kernel.org, rppt@linux.ibm.com,
        vbabka@suse.cz
References: <20190418182321.GJ3040@uranus.lan>
 <20190430081844.22597-1-mkoutny@suse.com>
 <20190430081844.22597-2-mkoutny@suse.com>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <4c79fb09-c310-4426-68f7-8b268100359a@virtuozzo.com>
Date:   Tue, 30 Apr 2019 12:09:57 +0300
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On 30.04.2019 11:18, Michal Koutný 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.
> 
> Fixes: 88aa7cc688d4 ("mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct")
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Mateusz Guzik <mguzik@redhat.com>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> ---
>  mm/util.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 43a2984bccaa..5cf0e84a0823 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;

This looks OK for me.

But speaking about existing code it's a secret for me, why we ignore arg_lock
in binfmt code, e.g. in load_elf_binary().