[PATCH] mm: fix protection of mm_struct fields in get_cmdline()

[Konstantin Khlebnikov]

Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect arg_start|
end and env_start|end in mm_struct") related mm fields are protected with
separate spinlock and mmap_sem held for read is not enough for protection.

Fixes: 88aa7cc688d4 ("mm: introduce arg_lock to protect arg_start|end and 
env_start|end in mm_struct")
Signed-off-by: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
---
 mm/util.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/util.c b/mm/util.c
index e2e4f8c3fa12..540e7c157cf2 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -717,12 +717,12 @@ 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;