Re: [PATCH v4 1/4] lazy tlb: introduce lazy mm refcount helper functions

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Andrew Morton's message of June 8, 2021 11:48 am:
> On Tue, 08 Jun 2021 11:39:56 +1000 Nicholas Piggin <npiggin@gmail.com> wrote:
> 
>> > Looks like a functional change.  What's happening here?
>> 
>> That's kthread_use_mm being clever about the lazy tlb mm. If it happened 
>> that the kthread had inherited a the lazy tlb mm that happens to be the 
>> one we want to use here, then we already have a refcount to it via the 
>> lazy tlb ref.
>> 
>> So then it doesn't have to touch the refcount, but rather just converts
>> it from the lazy tlb ref to the returned reference. If the lazy tlb mm
>> doesn't get a reference, we can't do that.
> 
> Please cover this in the changelog and perhaps a code comment.
> 

Yeah fair enough, I'll even throw in a bug fix as well (your nose was right, 
and it was too clever for me by half...)

Thanks,
Nick

--
Fix a refcounting bug in kthread_use_mm (the mm reference is increased
unconditionally now, but the lazy tlb refcount is still only dropped only
if mm != active_mm).

And an update for the changelog:

If a kernel thread's current lazy tlb mm happens to be the one it wants to
use, then kthread_use_mm() cleverly transfers the mm refcount from the
lazy tlb mm reference to the returned reference. If the lazy tlb mm
reference is no longer identical to a normal reference, this trick does not
work, so that is changed to be explicit about the two references.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 kernel/kthread.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index b70e28431a01..5e9797b2d06e 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1314,6 +1314,11 @@ void kthread_use_mm(struct mm_struct *mm)
 	WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
 	WARN_ON_ONCE(tsk->mm);
 
+	/*
+	 * It's possible that tsk->active_mm == mm here, but we must
+	 * still mmgrab(mm) and mmdrop_lazy_tlb(active_mm), because lazy
+	 * mm may not have its own refcount (see mmgrab/drop_lazy_tlb()).
+	 */
 	mmgrab(mm);
 
 	task_lock(tsk);
@@ -1338,12 +1343,9 @@ void kthread_use_mm(struct mm_struct *mm)
 	 * memory barrier after storing to tsk->mm, before accessing
 	 * user-space memory. A full memory barrier for membarrier
 	 * {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by
-	 * mmdrop(), or explicitly with smp_mb().
+	 * mmdrop_lazy_tlb().
 	 */
-	if (active_mm != mm)
-		mmdrop_lazy_tlb(active_mm);
-	else
-		smp_mb();
+	mmdrop_lazy_tlb(active_mm);
 
 	to_kthread(tsk)->oldfs = force_uaccess_begin();
 }
-- 
2.23.0