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Subject[PATCH] 4.4.86-rt99: fix sync breakage between nr_cpus_allowed and cpus_allowed
4.4.86-rt99's patch


introduces a place where a task's cpus_allowed mask is
updated without a corresponding update to nr_cpus_allowed.

This path is executed when task affinity is changed while
migrate_disabled() is true. As there is no code present
to set nr_cpus_allowed when the migrate_disable state is
dropped, the scheduler at that point on may make incorrect
scheduling decisions for this task.

My testing consists of temporarily adding a

if (tsk_nr_cpus_allowed(p) == cpumask_weight(tsk_cpus_allowed(p))

stmt to schedule() and running a simple affinity rotation
program I wrote, one that rotates the threads of stress(1).
While rotating, I got the expected kernel error messages.
With this patch applied the messages disappeared.

Signed-off-by: Joe Korty <>

Index: b/kernel/sched/core.c
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1220,6 +1220,7 @@ void do_set_cpus_allowed(struct task_str

if (__migrate_disabled(p)) {
+ p->nr_cpus_allowed = cpumask_weight(new_mask);
cpumask_copy(&p->cpus_allowed, new_mask);
 \ /
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