[PATCH -mmotm] prctl: add comment about mmap_sem and arg_lock

[Yang Shi]

Add comment to elaborate why mmap_sem for is used by prctl.

Signed-off-by: Yang Shi <yang....@linux.alibaba.com>
---
akpm: this patch can be foled into:
mm-introduce-arg_lock-to-protect-arg_startend-and-env_startend-in-mm_struct.patch

 kernel/sys.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sys.c b/kernel/sys.c
index 0cc5a1c..943fdc5 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2011,6 +2011,10 @@ static int prctl_set_mm_map(int opt, const void __user 
*addr, unsigned long data
                        return error;
        }
 
+       /*
+        * arg_lock protects concurent updates but we still need mmap_sem for
+        * read to exclude races with sys_brk.
+        */
        down_read(&mm->mmap_sem);
 
        /*
-- 
1.8.3.1