On Tue, Jul 21, 2020 at 02:13:08PM +0200, pet...@infradead.org wrote:
>
> There is apparently one site that violates the rule that only current
> and ttwu() will modify task->state, namely ptrace_{,un}freeze_traced()
> will change task->state for a remote task.
>
> Oleg explains:
>
> "TASK_TRACED/TASK_STOPPED was always protected by siglock. In
> particular, ttwu(__TASK_TRACED) must be always called with siglock
> held. That is why ptrace_freeze_traced() assumes it can safely do
> s/TASK_TRACED/__TASK_TRACED/ under spin_lock(siglock)."
>
> This breaks the ordering scheme introduced by commit:
>
> dbfb089d360b ("sched: Fix loadavg accounting race")
>
> Specifically, the reload not matching no longer implies we don't have
> to block.
>
> Simply things by noting that what we need is a LOAD->STORE ordering
> and this can be provided by a control dependency.
>
> So replace:
>
> prev_state = prev->state;
> raw_spin_lock(&rq->lock);
> smp_mb__after_spinlock(); /* SMP-MB */
> if (... && prev_state && prev_state == prev->state)
> deactivate_task();
>
> with:
>
> prev_state = prev->state;
> if (... && prev_state) /* CTRL-DEP */
> deactivate_task();
>
> Since that already implies the 'prev->state' load must be complete
> before allowing the 'prev->on_rq = 0' store to become visible.
>
> Fixes: dbfb089d360b ("sched: Fix loadavg accounting race")
> Reported-by: Jiri Slaby <jirisl...@kernel.org>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> Tested-by: Paul Gortmaker <paul.gortma...@windriver.com>
> ---
Thank you. I applied this on top of v5.8-rc6 and re-ran the strace-test
suite successfully. So at least
Tested-by: Christian Brauner <christian.brau...@ubuntu.com>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4193,9 +4193,6 @@ static void __sched notrace __schedule(b
> local_irq_disable();
> rcu_note_context_switch(preempt);
>
> - /* See deactivate_task() below. */
> - prev_state = prev->state;
> -
> /*
> * Make sure that signal_pending_state()->signal_pending() below
> * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
> @@ -4219,11 +4216,16 @@ static void __sched notrace __schedule(b
> update_rq_clock(rq);
>
> switch_count = &prev->nivcsw;
> +
> /*
> - * We must re-load prev->state in case ttwu_remote() changed it
> - * before we acquired rq->lock.
> + * We must load prev->state once (task_struct::state is volatile), such
> + * that:
> + *
> + * - we form a control dependency vs deactivate_task() below.
> + * - ptrace_{,un}freeze_traced() can change ->state underneath us.
> */
> - if (!preempt && prev_state && prev_state == prev->state) {
> + prev_state = prev->state;
> + if (!preempt && prev_state) {
> if (signal_pending_state(prev_state, prev)) {
> prev->state = TASK_RUNNING;
> } else {
> @@ -4237,10 +4239,12 @@ static void __sched notrace __schedule(b
>
> /*
> * __schedule() ttwu()
> - * prev_state = prev->state; if
> (READ_ONCE(p->on_rq) && ...)
> - * LOCK rq->lock goto out;
> - * smp_mb__after_spinlock();
> smp_acquire__after_ctrl_dep();
> - * p->on_rq = 0; p->state =
> TASK_WAKING;
> + * if (prev_state) if (p->on_rq && ...)
> + * p->on_rq = 0; goto out;
> + *
> smp_acquire__after_ctrl_dep();
> + * p->state = TASK_WAKING
> + *
> + * Where __schedule() and ttwu() have matching control
> dependencies.
> *
> * After this, schedule() must not care about p->state
> any more.
> */
