The following commit has been merged into the x86/cache branch of tip: Commit-ID: e0ad6dc8969f790f14bddcfd7ea284b7e5f88a16 Gitweb: https://git.kernel.org/tip/e0ad6dc8969f790f14bddcfd7ea284b7e5f88a16 Author: Reinette Chatre <reinette.chatre@xxxxxxxxx> AuthorDate: Thu, 17 Dec 2020 14:31:20 -08:00 Committer: Borislav Petkov <bp@xxxxxxx> CommitterDate: Mon, 11 Jan 2021 11:34:45 +01:00 x86/resctrl: Use task_curr() instead of task_struct->on_cpu to prevent unnecessary IPI 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-f4b1-9462205fd581@xxxxxxx Reported-by: James Morse <james.morse@xxxxxxx> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Link: https://lkml.kernel.org/r/e9e68ce1441a73401e08b641cc3b9a3cf13fe6d4.1608243147.git.reinette.chatre@xxxxxxxxx --- 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 460f3e0..37f37df 100644 --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c @@ -2313,19 +2313,15 @@ 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);
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