Re: [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Michal Hocko]

On Tue 10-04-18 14:02:42, Cyrill Gorcunov wrote:
> On Tue, Apr 10, 2018 at 12:42:15PM +0200, Michal Hocko wrote:
> > On Tue 10-04-18 12:40:47, Cyrill Gorcunov wrote:
> > > On Tue, Apr 10, 2018 at 11:09:17AM +0200, Michal Hocko wrote:
> > > > On Tue 10-04-18 05:52:54, Yang Shi wrote:
> > > > [...]
> > > > > So, introduce a new spinlock in mm_struct to protect the concurrent
> > > > > access to arg_start|end, env_start|end and others except start_brk and
> > > > > brk, which are still protected by mmap_sem to avoid concurrent access
> > > > > from do_brk().
> > > > 
> > > > Is there any fundamental problem with brk using the same lock?
> > > 
> > > Seems so. Look into mm/mmap.c:brk syscall which reads and writes
> > > brk value under mmap_sem ('cause of do_brk called inside).
> > 
> > Why cannot we simply use the lock when the value is updated?
> 
> Because do_brk does vma manipulations, for this reason it's
> running under down_write_killable(&mm->mmap_sem). Or you
> mean something else?

Yes, all we need the new lock for is to get a consistent view on brk
values. I am simply asking whether there is something fundamentally
wrong by doing the update inside the new lock while keeping the original
mmap_sem locking in the brk path. That would allow us to drop the
mmap_sem lock in the proc path when looking at brk values.

-- 
Michal Hocko
SUSE Labs