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Subject[PATCH] sched: Fix race against ptrace_freeze_trace()

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 <jirislaby@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.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.
*/
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