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Date:   Thu, 2 May 2019 23:57:34 +0300
From:   Cyrill Gorcunov <gorcunov@gmail.com>
To:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Cc:     akpm@linux-foundation.org, arunks@codeaurora.org, brgl@bgdev.pl,
        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, ktkhai@virtuozzo.com
Subject: Re: [PATCH v3 2/2] prctl_set_mm: downgrade mmap_sem to read lock
Message-ID: <20190502205734.GE2488@uranus.lan>
References: <0a48e0a2-a282-159e-a56e-201fbc0faa91@virtuozzo.com>
 <20190502125203.24014-1-mkoutny@suse.com>
 <20190502125203.24014-3-mkoutny@suse.com>
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On Thu, May 02, 2019 at 02:52:03PM +0200, Michal Koutn? 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. But there
> still remained two places that (mis)use mmap_sem.
> 
> get_cmdline should also use arg_lock instead of mmap_sem when it reads the
> boundaries.
> 
> The second place that should use arg_lock is in prctl_set_mm. By
> protecting the boundaries fields with the arg_lock, we can downgrade
> mmap_sem to reader lock (analogous to what we already do in
> prctl_set_mm_map).
> 
> v2: call find_vma without arg_lock held
> v3: squashed get_cmdline arg_lock patch
> 
> 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>
> CC: Cyrill Gorcunov <gorcunov@gmail.com>
> Co-developed-by: Laurent Dufour <ldufour@linux.ibm.com>
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> Signed-off-by: Michal Koutn? <mkoutny@suse.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>