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Commit 22c4eeaf authored by Reinette Chatre's avatar Reinette Chatre Committed by Greg Kroah-Hartman
Browse files

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@arm.com



Reported-by: default avatarJames Morse <james.morse@arm.com>
Signed-off-by: default avatarReinette Chatre <reinette.chatre@intel.com>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/e9e68ce1441a73401e08b641cc3b9a3cf13fe6d4.1608243147.git.reinette.chatre@intel.com


Stable-dep-of: fe1f0714385f ("x86/resctrl: Fix task CLOSID/RMID update race")
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent b3f92424

arch/x86/kernel/cpu/resctrl/rdtgroup.c

+5 −9
Original line number Diff line number Diff line
@@ -2178,19 +2178,15 @@ 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);