Remove the open-coded mm switching in kthread_use_mm() and
kthread_unuse_mm().
This has one internally-visible effect: the old code active_mm
refcounting was inconsistent with everything else and mmgrabbed the
mm in kthread_use_mm(). The new code refcounts following the same
rules as normal user threads, so kthreads that are currently using
a user mm will not hold an mm_count reference.
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
kernel/kthread.c | 45 ++-------------------------------------------
1 file changed, 2 insertions(+), 43 deletions(-)
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 18b0a2e0e3b2..77586f5b14e5 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1344,37 +1344,12 @@ EXPORT_SYMBOL(kthread_destroy_worker);
*/
void kthread_use_mm(struct mm_struct *mm)
{
- struct mm_struct *active_mm;
struct task_struct *tsk = current;
WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
WARN_ON_ONCE(tsk->mm);
- task_lock(tsk);
- /*
- * membarrier() requires a full barrier before switching mm.
- */
- smp_mb__after_spinlock();
-
- /* Hold off tlb flush IPIs while switching mm's */
- local_irq_disable();
- active_mm = tsk->active_mm;
- if (active_mm != mm) {
- mmgrab(mm);
- tsk->active_mm = mm;
- }
- WRITE_ONCE(tsk->mm, mm); /* membarrier reads this without locks */
- membarrier_update_current_mm(mm);
- switch_mm_irqs_off(active_mm, mm, tsk);
- membarrier_finish_switch_mm(mm);
- local_irq_enable();
- task_unlock(tsk);
-#ifdef finish_arch_post_lock_switch
- finish_arch_post_lock_switch();
-#endif
-
- if (active_mm != mm)
- mmdrop(active_mm);
+ __change_current_mm(mm, false);
to_kthread(tsk)->oldfs = force_uaccess_begin();
}
@@ -1393,23 +1368,7 @@ void kthread_unuse_mm(struct mm_struct *mm)
force_uaccess_end(to_kthread(tsk)->oldfs);
- task_lock(tsk);
- /*
- * When a kthread stops operating on an address space, the loop
- * in membarrier_{private,global}_expedited() may not observe
- * that tsk->mm, and not issue an IPI. Membarrier requires a
- * memory barrier after accessing user-space memory, before
- * clearing tsk->mm.
- */
- smp_mb__after_spinlock();
- sync_mm_rss(mm);
- local_irq_disable();
- WRITE_ONCE(tsk->mm, NULL); /* membarrier reads this without locks */
- membarrier_update_current_mm(NULL);
- /* active_mm is still 'mm' */
- enter_lazy_tlb(mm, tsk);
- local_irq_enable();
- task_unlock(tsk);
+ __change_current_mm_to_kernel();
}
EXPORT_SYMBOL_GPL(kthread_unuse_mm);
--
2.33.1
