On 10/10/23 15:44, Waiman Long wrote:
On 10/10/23 01:34, Juri Lelli wrote:
Hi,
On 09/10/23 15:15, Waiman Long wrote:
The nr_deadline_tasks field in cpuset structure was introduced by
commit 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task
in cpusets"). Unlike nr_migrate_dl_tasks which is only modified under
cpuset_mutex, nr_deadline_tasks can be updated in various contexts
under different locks. As a result, data races may happen that cause
incorrect value to be stored in nr_deadline_tasks leading to incorrect
Could you please make an example of such data races?
Since update to cs->nr_deadline_tasks is not protected by a single
lock, it is possible that multiple CPUs may try to modify it at the
same time. It is possible that nr_deadline_tasks++ and
nr_deadline_tasks-- can be done in a single instruction like in x86
and hence atomic. However, operation like "cs->nr_deadline_tasks +=
cs->nr_migrate_dl_tasks" is likely a RMW operation and so is subjected
to racing. It is mostly theoretical, but probably not impossible.
Sorry, even increment and decrement operators are not atomic.
inc_dl_tasks_cs() is only called from switched_to_dl() in deadline.c
which is protected by the rq_lock, but there are multiple rq's.
dec_dl_tasks_cs() is called from switched_from_dl() in deadline.c and
cgroup_exit() in cgroup.c. The later one is protected by css_set_lock.
The other place where nr_deadline_tasks can be changed is in
cpuset_attach() protected by cpuset_mutex.
Cheers,
Longman
