rcu-tasks: Don't delete holdouts within trc_inspect_reader()

As Yanfei pointed out, although invoking trc_del_holdout() is safe
from the viewpoint of the integrity of the holdout list itself,
the put_task_struct() invoked by trc_del_holdout() can result in
use-after-free errors due to later accesses to this task_struct structure
by the RCU Tasks Trace grace-period kthread.

This commit therefore removes this call to trc_del_holdout() from
trc_inspect_reader() in favor of the grace-period thread's existing call
to trc_del_holdout(), thus eliminating that particular class of
use-after-free errors.

Reported-by: "Xu, Yanfei" <yanfei.xu@windriver.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 03a118d..3d5cb6c 100644 (file)
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -953,10 +953,9 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
                in_qs = likely(!t->trc_reader_nesting);
        }
 
-       // Mark as checked.  Because this is called from the grace-period
-       // kthread, also remove the task from the holdout list.
+       // Mark as checked so that the grace-period kthread will
+       // remove it from the holdout list.
        t->trc_reader_checked = true;
-       trc_del_holdout(t);
 
        if (in_qs)
                return true;  // Already in quiescent state, done!!!