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

[Michal Koutný]

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 <gorcu...@gmail.com>
CC: Laurent Dufour <lduf...@linux.ibm.com>
Signed-off-by: Michal Koutný <mkou...@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;
 }
 
-- 
2.16.4