Re: [PATCH 1/3] [PATCH i386] during VM oom condition, kill all threads in process group

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 08 Jun 2007 14:19:18 -0500
Will Schmidt <will_schmidt@vnet.ibm.com> wrote:

> > > > zap_other_threads() requires tasklist_lock.
> 
> Yup, I missed that.   Thanks for pointing it out.
> 
> > > > 
> > > > If we're going to do this then we should probably create some new function
> > > > (with a better name) which takes tasklsit_lock and then calls
> > > > zap_other_threads().
> 
> I expect this will be a write_lock_irq() since zap_other_threads will be
> doing a bit more than just reading the task info.

No, I think read_lock() will be sufficient.

In fact, it's probably the case that rcu_read_lock() is now sufficient
locking coverage for zap_other_threads() (cc's people).

It had better be, because do_group_exit() forgot to take tasklist_lock.  It
is perhaps relying upon spin_lock()'s hidden rcu_read_lock() properties
without so much as a code comment, which would be somewhat nasty of it.

You could perhaps just call do_group_exit() from within the fault handler,
btw.

> This will be down in a do-page-fault failure path (see
> arch/*/mm/fault.c).  I wonder if calling write_lock is going to be safe,
> or if its possible to get into a deadlock?  i.e. should I branch back up
> to the survive: label if I can't take the lock?  Would that even be
> sufficient? or is it not an issue here? 

You can take the lock in the fault handler.  Nobody should be getting
pagefaults while holding tasklist_lock.  (Well, a vmalloc fault might, but
that's a special-case which doesn't allocate memory or anything like that).