Patch "x86/resctrl: Use task_curr() instead of task_struct->on_cpu to prevent unnecessary IPI" has been added to the 4.19-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    x86/resctrl: Use task_curr() instead of task_struct->on_cpu to prevent unnecessary IPI

to the 4.19-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-resctrl-use-task_curr-instead-of-task_struct-on_.patch
and it can be found in the queue-4.19 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit f9bd347aa46b67363257b15846bc67cf43da7877
Author: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Date:   Thu Dec 17 14:31:20 2020 -0800

    x86/resctrl: Use task_curr() instead of task_struct->on_cpu to prevent unnecessary IPI
    
    [ Upstream commit e0ad6dc8969f790f14bddcfd7ea284b7e5f88a16 ]
    
    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
    Stable-dep-of: fe1f0714385f ("x86/resctrl: Fix task CLOSID/RMID update race")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
index f406e3b85bdb..8c405149c671 100644
--- a/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
+++ b/arch/x86/kernel/cpu/intel_rdt_rdtgroup.c
@@ -2140,19 +2140,15 @@ 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);