Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Hyunwoo Kim]

On Wed, Aug 26, 2026 at 02:55:31PM +0200, Oleg Nesterov wrote:
> OMG, my head spins ;) I'll try to read your explanation tomorrow.
> Right now I am all confused.
> 
> But let me ask some stupid questions right now, please help me to
> understand the problem.
> 
> On 08/26, Hyunwoo Kim wrote:
> >
> > begin_new_exec() calls posix_cpu_timers_exit(me) right after
> > exec_task_namespaces() and that removes the leftover node, so the state
> > normally stays invisible. But bprm->point_of_no_return is set before
> > de_thread(), so if unshare_files(), set_mm_exe_file(), exec_mmap() or
> > exec_task_namespaces() fails, the task dies before it gets there.
> 
> Do I understand this correctly? If begin_new_exec() does call
> posix_cpu_timers_exit(me), then everything is fine.

Yes. A failure after it, e.g. unshare_sighand(), is harmless.

> 
> Yes? If yes
> 
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -1003,6 +1003,18 @@ static int de_thread(struct task_struct *tsk)
> >  		 * the former thread group leader:
> >  		 */
> >
> > +#ifdef CONFIG_POSIX_TIMERS
> > +		/*
> > +		 * exchange_tids() hands this thread's PID to the old leader,
> > +		 * which is reaped right after. The PID lookup in
> > +		 * timer_lock_sighand() then fails while the per thread CPU
> > +		 * timers are still queued here, so dequeue them first.
> > +		 */
> > +		spin_lock(lock);
> > +		posix_cpu_timers_exit(tsk);
> > +		spin_unlock(lock);
> > +#endif
> 
> ... then why do we need to call posix_cpu_timers_exit(current) before
> exchange_tids() ?

Strictly we don't. posix_cpu_timers_exit() walks tsk->posix_cputimers
directly and never does the PID lookup, so it only has to happen before
exit_itimers()

I put it before exchange_tids() so that the state timer_lock_sighand()
warns about never exists in the first place.

> 
> And what if another process attaches another cpu timer to the execing
> thread right after the code above?

Not for a per-thread timer, pid_for_clock() requires same_thread_group()
and there is no other thread left at that point.

A process wide timer has no such check, but it goes to
signal->posix_cputimers and the TGID lookup stays valid, so it isn't
affected.

> 
> Can we move posix_cpu_timers_exit() or the whole CONFIG_POSIX_TIMERS
> sequence in begin_new_exec() up, right after de_thread() before the
> "if (retval)" check?

posix_cpu_timers_exit(me) there is enough. I haven't tested moving the 
whole block.


Best regards,
Hyunwoo Kim