Re: [PATCH 3/3] prctl_set_mm: downgrade mmap_sem to read lock

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 30.04.2019 11:18, Michal Koutný wrote:
> Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect
> arg_start|end and env_start|end in mm_struct") we use arg_lock for
> boundaries modifications. Synchronize prctl_set_mm with this lock and
> keep mmap_sem for reading only (analogous to what we already do in
> prctl_set_mm_map).
> 
> v2: call find_vma without arg_lock held
> 
> CC: Cyrill Gorcunov <gorcunov@gmail.com>
> CC: Laurent Dufour <ldufour@linux.ibm.com>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> ---
>  kernel/sys.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index e1acb444d7b0..641fda756575 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2123,9 +2123,14 @@ static int prctl_set_mm(int opt, unsigned long addr,
>  
>  	error = -EINVAL;
>  
> -	down_write(&mm->mmap_sem);
> +	/*
> +	 * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for
> +	 * a) concurrent sys_brk, b) finding VMA for addr validation.
> +	 */
> +	down_read(&mm->mmap_sem);
>  	vma = find_vma(mm, addr);
>  
> +	spin_lock(&mm->arg_lock);
>  	prctl_map.start_code	= mm->start_code;
>  	prctl_map.end_code	= mm->end_code;
>  	prctl_map.start_data	= mm->start_data;
> @@ -2213,7 +2218,8 @@ static int prctl_set_mm(int opt, unsigned long addr,
>  
>  	error = 0;
>  out:
> -	up_write(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
> +	up_read(&mm->mmap_sem);
>  	return error;

Hm, shouldn't spin_lock()/spin_unlock() pair go as a fixup to existing code
in a separate patch? 

Without them, the existing code has a problem at least in get_mm_cmdline().