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:
dbfb089d36 ("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: dbfb089d36 ("sched: Fix loadavg accounting race")
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Tested-by: Christian Brauner <christian.brauner@ubuntu.com>
d136122f58