Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once

Paul Menage
2009-08-06T10:28:32Z

On Thu, Aug 6, 2009 at 2:58 AM, Louis Rilling<Louis.Rilling@kerlabs.com> wrote:
>
> mutex_lock_nested is not enough, since this would require putting each thread's
> mutex in a different class. Again, something like mutex_lock_nest_lock() is
> the solution, especially since Peter's recent improvement.
>

OK, well if lockdep can't currently handle the "writer takes a lock on
every thread" model, then maybe we should go with a simpler model
until someone shows a performance issue with it? Ben's original
patches had a per-task_struct lock, and a thread forking with CLONE_VM
would down_read() its group leader's lock. Something that's even
simpler (doesn't have to deal with thread group leader changing due to
an execve()), and avoids the per-task_struct overhead would be to put
the lock in sighand_struct instead (so only one per process). The
procs file writer does a down_write(&tsk->sighand->fork_sem), and
cgroup_fork() can do a down_read(&current->sighand->fork_sem) if
flags&CLONE_SIGHAND.

If you put it as the second member of sighand_struct, there wouldn't
even be any extra cacheline bouncing in the common case, since
copy_sighand() would already have brought that line into cache in
order to do atomic_inc(&current->sighand->count)

Paul