[PATCH 6/6] sched: Simplify set_affinity_pending refcounts

Peter Zijlstra
2021-02-24T14:42:18Z

Now that we have set_affinity_pending::stop_pending to indicate if a
stopper is in progress, and we have the guarantee that if that stopper
exists, it will (eventually) complete our @pending we can simplify the
refcount scheme by no longer counting the stopper thread.

Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/core.c | 32 ++++++++++++++++++++------------
1 file changed, 20 insertions(+), 12 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1862,6 +1862,10 @@ struct migration_arg {
struct set_affinity_pending *pending;
};

+/*
+ * @refs: number of wait_for_completion()
+ * @stop_pending: is @stop_work in use
+ */
struct set_affinity_pending {
refcount_t refs;
unsigned int stop_pending;
@@ -1997,10 +2001,6 @@ static int migration_cpu_stop(void *data
if (complete)
complete_all(&pending->done);

- /* For pending->{arg,stop_work} */
- if (pending && refcount_dec_and_test(&pending->refs))
- wake_up_var(&pending->refs);
-
return 0;
}

@@ -2199,12 +2199,16 @@ static int affine_move_task(struct rq *r
push_task = get_task_struct(p);
}

+ /*
+ * If there are pending waiters, but no pending stop_work,
+ * then complete now.
+ */
pending = p->migration_pending;
- if (pending) {
- refcount_inc(&pending->refs);
+ if (pending && !pending->stop_pending) {
p->migration_pending = NULL;
complete = true;
}
+
task_rq_unlock(rq, p, rf);

if (push_task) {
@@ -2213,7 +2217,7 @@ static int affine_move_task(struct rq *r
}

if (complete)
- goto do_complete;
+ complete_all(&pending->done);

return 0;
}
@@ -2264,9 +2268,9 @@ static int affine_move_task(struct rq *r
if (!stop_pending)
pending->stop_pending = true;

- refcount_inc(&pending->refs); /* pending->{arg,stop_work} */
if (flags & SCA_MIGRATE_ENABLE)
p->migration_flags &= ~MDF_PUSH;
+
task_rq_unlock(rq, p, rf);

if (!stop_pending) {
@@ -2282,12 +2286,13 @@ static int affine_move_task(struct rq *r
if (task_on_rq_queued(p))
rq = move_queued_task(rq, rf, p, dest_cpu);

- p->migration_pending = NULL;
- complete = true;
+ if (!pending->stop_pending) {
+ p->migration_pending = NULL;
+ complete = true;
+ }
}
task_rq_unlock(rq, p, rf);

-do_complete:
if (complete)
complete_all(&pending->done);
}
@@ -2295,7 +2300,7 @@ static int affine_move_task(struct rq *r
wait_for_completion(&pending->done);

if (refcount_dec_and_test(&pending->refs))
- wake_up_var(&pending->refs);
+ wake_up_var(&pending->refs); /* No UaF, just an address */

/*
* Block the original owner of &pending until all subsequent callers
@@ -2303,6 +2308,9 @@ static int affine_move_task(struct rq *r
*/
wait_var_event(&my_pending.refs, !refcount_read(&my_pending.refs));

+ /* ARGH */
+ WARN_ON_ONCE(my_pending.stop_pending);
+
return 0;
}