sched: Simplify set_affinity_pending refcounts

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()")
Cc: stable@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224131355.724130207@infradead.org

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4e4d100..9819121 100644 (file)
--- 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 @@ out:
        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 *rq, struct task_struct *p, struct rq_flag
                        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 *rq, struct task_struct *p, struct rq_flag
                }
 
                if (complete)
-                       goto do_complete;
+                       complete_all(&pending->done);
 
                return 0;
        }
@@ -2264,9 +2268,9 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
                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 *rq, struct task_struct *p, struct rq_flag
                        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 @@ do_complete:
        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 @@ do_complete:
         */
        wait_var_event(&my_pending.refs, !refcount_read(&my_pending.refs));
 
+       /* ARGH */
+       WARN_ON_ONCE(my_pending.stop_pending);
+
        return 0;
 }