Commit a2496049 authored by Nicholas Piggin's avatar Nicholas Piggin Committed by Michael Ellerman
Browse files

powerpc/64s/radix: add warning and comments in mm_cpumask trim 



Add a comment explaining part of the logic for mm_cpumask trimming, and
add a (hopefully graceful) check and warning in case something gets it
wrong.

Signed-off-by: default avatarNicholas Piggin <npiggin@gmail.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201217134731.488135-2-npiggin@gmail.com
parent e79b76e0

arch/powerpc/mm/book3s64/radix_tlb.c

+21 −6
Original line number Diff line number Diff line
@@ -653,13 +653,14 @@ static void do_exit_flush_lazy_tlb(void *arg) 
{

	struct mm_struct *mm = arg;

	unsigned long pid = mm->context.id;

	int cpu = smp_processor_id();



	/*

	 * A kthread could have done a mmget_not_zero() after the flushing CPU

	 * checked mm_is_singlethreaded, and be in the process of

	 * kthread_use_mm when interrupted here. In that case, current->mm will

	 * be set to mm, because kthread_use_mm() setting ->mm and switching to

	 * the mm is done with interrupts off.

	 * checked mm_cpumask, and be in the process of kthread_use_mm when

	 * interrupted here. In that case, current->mm will be set to mm,

	 * because kthread_use_mm() setting ->mm and switching to the mm is

	 * done with interrupts off.

	 */

	if (current->mm == mm)

		goto out_flush;
@@ -673,8 +674,22 @@ static void do_exit_flush_lazy_tlb(void *arg) 
		mmdrop(mm);

	}



	/*

	 * This IPI is only initiated from a CPU which is running mm which

	 * is a single-threaded process, so there will not be another racing

	 * IPI coming in where we would find our cpumask already clear.

	 *

	 * Nothing else clears our bit in the cpumask except CPU offlining,

	 * in which case we should not be taking IPIs here. However check

	 * this just in case the logic is wrong somewhere, and don't underflow

	 * the active_cpus count.

	 */

	if (cpumask_test_cpu(cpu, mm_cpumask(mm))) {

		atomic_dec(&mm->context.active_cpus);

	cpumask_clear_cpu(smp_processor_id(), mm_cpumask(mm));

		cpumask_clear_cpu(cpu, mm_cpumask(mm));

	} else {

		WARN_ON_ONCE(1);

	}



out_flush:

	_tlbiel_pid(pid, RIC_FLUSH_ALL);