Re: [PATCH v3] sched/cputime: Protect some other sum_exec_runtime reads on 32 bit cpus

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 07, 2016 at 08:38:27AM +0200, Stanislaw Gruszka wrote:
> On Tue, Sep 06, 2016 at 09:01:29PM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 06, 2016 at 02:49:08PM +0200, Stanislaw Gruszka wrote:
> > > diff --git a/kernel/exit.c b/kernel/exit.c
> > > index 2f974ae..a46f96f 100644
> > > --- a/kernel/exit.c
> > > +++ b/kernel/exit.c
> > > @@ -134,7 +134,7 @@ static void __exit_signal(struct task_struct *tsk)
> > >  	sig->inblock += task_io_get_inblock(tsk);
> > >  	sig->oublock += task_io_get_oublock(tsk);
> > >  	task_io_accounting_add(&sig->ioac, &tsk->ioac);
> > > -	sig->sum_sched_runtime += tsk->se.sum_exec_runtime;
> > > +	sig->sum_sched_runtime += read_sum_exec_runtime(tsk);
> > >  	sig->nr_threads--;
> > >  	__unhash_process(tsk, group_dead);
> > >  	write_sequnlock(&sig->stats_lock);
> > 
> > If I'm not mistaken, at this point @p is dead, sum_exec_runtime will not
> > ever be updated again.
> 
> I think at this point find_task_by_vpid() will still return not-null
> pointer, hence we can update sum_exec_runtime via:
> 
> posix_cpu_{clock_get(),timer_set(),timer_get()} ->
> cpu_clock_sample() -> task_sched_runtime() -> update_curr()
> 
> if some thread will do cpuclock/cputimer related syscall for
> different thread and CPUCLOCK_SCHED clock.

Ah, ok. I always forget how the exit path works..

> > > diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
> > > index 39008d7..a2d753b 100644
> > > --- a/kernel/time/posix-cpu-timers.c
> > > +++ b/kernel/time/posix-cpu-timers.c
> > > @@ -848,7 +848,7 @@ static void check_thread_timers(struct task_struct *tsk,
> > >  	tsk_expires->virt_exp = expires_to_cputime(expires);
> > >  
> > >  	tsk_expires->sched_exp = check_timers_list(++timers, firing,
> > > -						   tsk->se.sum_exec_runtime);
> > > +						   read_sum_exec_runtime(tsk));
> > >  
> > >  	/*
> > >  	 * Check for the special case thread timers.
> > 
> > @tsk == current and IRQs are disabled, sum_exec_runtime cannot be
> > updated.
> 
> It still can be updated via syscalls on different thread, same paths
> as above.

Yep, realized that over-night.