+ lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch added to -mm tree

+ lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch added to -mm tree

[akpm]

The patch titled
     Subject: lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix
has been added to the -mm tree.  Its filename is
     lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch

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------------------------------------------------------
From: Nicholas Piggin <npiggin@gmail.com>
Subject: lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix

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).

Link: https://lkml.kernel.org/r/1623125298.bx63h3mopj.astroid@bobo.none
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/kthread.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/kernel/kthread.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix
+++ a/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();
 }
_

Patches currently in -mm which might be from npiggin@gmail.com are

lazy-tlb-introduce-lazy-mm-refcount-helper-functions.patch
lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch
lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable.patch
lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option.patch
powerpc-64s-enable-mmu_lazy_tlb_shootdown.patch

Re: + lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch added to -mm tree

[Stephen Rothwell]

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Hi all,

On Mon, 07 Jun 2021 21:15:48 -0700 akpm@linux-foundation.org wrote:
>
> The patch titled
>      Subject: lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix
> has been added to the -mm tree.  Its filename is
>      lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch

Added to linux-next today.

-- 
Cheers,
Stephen Rothwell

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