[tip: sched/urgent] sched: Fix race in task_call_func()

tip-bot2 for Peter Zijlstra
2022-11-14T09:10:27Z

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID: 91dabf33ae5df271da63e87ad7833e5fdb4a44b9
Gitweb: https://git.kernel.org/tip/91dabf33ae5df271da63e87ad7833e5fdb4a44b9
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 26 Oct 2022 13:43:00 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 14 Nov 2022 09:58:32 +01:00

sched: Fix race in task_call_func()

There is a very narrow race between schedule() and task_call_func().

CPU0 CPU1

__schedule()
rq_lock();
prev_state = READ_ONCE(prev->__state);
if (... && prev_state) {
deactivate_tasl(rq, prev, ...)
prev->on_rq = 0;

task_call_func()
raw_spin_lock_irqsave(p->pi_lock);
state = READ_ONCE(p->__state);
smp_rmb();
if (... || p->on_rq) // false!!!
rq = __task_rq_lock()

ret = func();

next = pick_next_task();
rq = context_switch(prev, next)
prepare_lock_switch()
spin_release(&__rq_lockp(rq)->dep_map...)

So while the task is on it's way out, it still holds rq->lock for a
little while, and right then task_call_func() comes in and figures it
doesn't need rq->lock anymore (because the task is already dequeued --
but still running there) and then the __set_task_frozen() thing observes
it's holding rq->lock and yells murder.

Avoid this by waiting for p->on_cpu to get cleared, which guarantees
the task is fully finished on the old CPU.

( While arguably the fixes tag is 'wrong' -- none of the previous
task_call_func() users appears to care for this case. )

Fixes: f5d39b020809 ("freezer,sched: Rewrite core freezer logic")
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lkml.kernel.org/r/Y1kdRNNfUeAU+FNl@hirez.programming.kicks-ass.net
---
kernel/sched/core.c | 52 +++++++++++++++++++++++++++++---------------
1 file changed, 35 insertions(+), 17 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cb2aa2b..daff72f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4200,6 +4200,40 @@ out:
return success;
}

+static bool __task_needs_rq_lock(struct task_struct *p)
+{
+ unsigned int state = READ_ONCE(p->__state);
+
+ /*
+ * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
+ * the task is blocked. Make sure to check @state since ttwu() can drop
+ * locks at the end, see ttwu_queue_wakelist().
+ */
+ if (state == TASK_RUNNING || state == TASK_WAKING)
+ return true;
+
+ /*
+ * Ensure we load p->on_rq after p->__state, otherwise it would be
+ * possible to, falsely, observe p->on_rq == 0.
+ *
+ * See try_to_wake_up() for a longer comment.
+ */
+ smp_rmb();
+ if (p->on_rq)
+ return true;
+
+#ifdef CONFIG_SMP
+ /*
+ * Ensure the task has finished __schedule() and will not be referenced
+ * anymore. Again, see try_to_wake_up() for a longer comment.
+ */
+ smp_rmb();
+ smp_cond_load_acquire(&p->on_cpu, !VAL);
+#endif
+
+ return false;
+}
+
/**
* task_call_func - Invoke a function on task in fixed state
* @p: Process for which the function is to be invoked, can be @current.
@@ -4217,28 +4251,12 @@ out:
int task_call_func(struct task_struct *p, task_call_f func, void *arg)
{
struct rq *rq = NULL;
- unsigned int state;
struct rq_flags rf;
int ret;

raw_spin_lock_irqsave(&p->pi_lock, rf.flags);

- state = READ_ONCE(p->__state);
-
- /*
- * Ensure we load p->on_rq after p->__state, otherwise it would be
- * possible to, falsely, observe p->on_rq == 0.
- *
- * See try_to_wake_up() for a longer comment.
- */
- smp_rmb();
-
- /*
- * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
- * the task is blocked. Make sure to check @state since ttwu() can drop
- * locks at the end, see ttwu_queue_wakelist().
- */
- if (state == TASK_RUNNING || state == TASK_WAKING || p->on_rq)
+ if (__task_needs_rq_lock(p))
rq = __task_rq_lock(p, &rf);

/*