James reported in [1] that there could be two tasks running on the same CPU with task_struct->on_cpu set. Using task_struct->on_cpu as a test if a task is running on a CPU may thus match the old task for a CPU while the scheduler is running and IPI it unnecessarily.
task_curr() is the correct helper to use. While doing so move the #ifdef check of the CONFIG_SMP symbol to be a C conditional used to determine if this helper should be used to ensure the code is always checked for correctness by the compiler. [1] https://lore.kernel.org/lkml/a782d2f3-d2f6-795f-f4b1-9462205fd...@arm.com Reported-by: James Morse <james.mo...@arm.com> Signed-off-by: Reinette Chatre <reinette.cha...@intel.com> --- V1->V2: * New patch in series arch/x86/kernel/cpu/resctrl/rdtgroup.c | 14 +++++--------- 1 file changed, 5 insertions(+), 9 deletions(-) diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c index 4042e1eb4f5d..9bd36210d220 100644 --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c @@ -2319,19 +2319,15 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to, t->closid = to->closid; t->rmid = to->mon.rmid; -#ifdef CONFIG_SMP /* - * This is safe on x86 w/o barriers as the ordering - * of writing to task_cpu() and t->on_cpu is - * reverse to the reading here. The detection is - * inaccurate as tasks might move or schedule - * before the smp function call takes place. In - * such a case the function call is pointless, but + * If the task is on a CPU, set the CPU in the mask. + * The detection is inaccurate as tasks might move or + * schedule before the smp function call takes place. + * In such a case the function call is pointless, but * there is no other side effect. */ - if (mask && t->on_cpu) + if (IS_ENABLED(CONFIG_SMP) && mask && task_curr(t)) cpumask_set_cpu(task_cpu(t), mask); -#endif } } read_unlock(&tasklist_lock); -- 2.26.2
