Re: [Qemu-devel] [RFC 1/2] linux-user: Don't leak cpus on thread exit

[David Gibson]

On Thu, Jul 14, 2016 at 10:52:48AM +0100, Peter Maydell wrote:
> On 14 July 2016 at 08:57, David Gibson <address@hidden> wrote:
> > Currently linux-user does not correctly clean up CPU instances properly
> > when running a threaded binary.
> >
> > On thread exit, do_syscall() removes the thread's CPU from the cpus list
> > and calls object_unref().  However, the CPU still is still referenced from
> > the QOM tree.  To correctly clean up we need to object_unparent() to remove
> > the CPU from the QOM tree, then object_unref() to release the final
> > reference we're holding.
> >
> > Once this is done, the explicit remove from the cpus list is no longer
> > necessary, since that's done automatically in the CPU unrealize path.
> >
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  linux-user/syscall.c | 7 ++-----
> >  1 file changed, 2 insertions(+), 5 deletions(-)
> >
> > I believe most full system targets also "leak" cpus in the same way,
> > except that since they don't support cpu hot unplug the cpus never
> > would have been disposed anyway.  I'll look into fixing that another
> > time.
> >
> > diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> > index 8bf6205..dd91791 100644
> > --- a/linux-user/syscall.c
> > +++ b/linux-user/syscall.c
> > @@ -6823,10 +6823,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
> > arg1,
> >          if (CPU_NEXT(first_cpu)) {
> >              TaskState *ts;
> >
> > -            cpu_list_lock();
> > -            /* Remove the CPU from the list.  */
> > -            QTAILQ_REMOVE(&cpus, cpu, node);
> > -            cpu_list_unlock();
> > +            object_unparent(OBJECT(cpu)); /* Remove from QOM */
> >              ts = cpu->opaque;
> >              if (ts->child_tidptr) {
> >                  put_user_u32(0, ts->child_tidptr);
> > @@ -6834,7 +6831,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
> > arg1,
> >                            NULL, NULL, 0);
> >              }
> >              thread_cpu = NULL;
> > -            object_unref(OBJECT(cpu));
> > +            object_unref(OBJECT(cpu)); /* Remove the last ref we're 
> > holding */
> 
> Is it OK to now be removing the CPU from the list after we've done
> the futex to signal the child task rather than before?

Ah.. not sure.  I was thinking the object_unparent() would trigger an
unrealize (which would do the list remove) even if there was a
reference keeping the object in existence.  I haven't confirmed that
thought.

It could obviously be fixed with an explicit unrealize before the
futex op.


> 
> >              g_free(ts);
> >              rcu_unregister_thread();
> >              pthread_exit(NULL);
> 
> thanks
> -- PMM
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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