linux-kernel - Re: [PATCH v3] mm: Fix kthread_use

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Message-Id: <20200721140623.4e8ecc6ef5d5ff42115d68fc@linux-foundation.org>
Date:   Tue, 21 Jul 2020 14:06:23 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, luto@...capital.net, axboe@...nel.dk,
        keescook@...omium.org, torvalds@...ux-foundation.org,
        jannh@...gle.com, will@...nel.org, hch@....de, npiggin@...il.com,
        mathieu.desnoyers@...icios.com
Subject: Re: [PATCH v3] mm: Fix kthread_use_mm() vs TLB invalidate

On Tue, 21 Jul 2020 17:41:06 +0200 Peter Zijlstra <peterz@...radead.org> wrote:

> 
> For SMP systems using IPI based TLB invalidation, looking at
> current->active_mm is entirely reasonable. This then presents the
> following race condition:
> 
> 
>   CPU0			CPU1
> 
>   flush_tlb_mm(mm)	use_mm(mm)
>     <send-IPI>
> 			  tsk->active_mm = mm;
> 			  <IPI>
> 			    if (tsk->active_mm == mm)
> 			      // flush TLBs
> 			  </IPI>
> 			  switch_mm(old_mm,mm,tsk);
> 
> 
> Where it is possible the IPI flushed the TLBs for @old_mm, not @mm,
> because the IPI lands before we actually switched.
> 
> Avoid this by disabling IRQs across changing ->active_mm and
> switch_mm().
> 
> [ There are all sorts of reasons this might be harmless for various
> architecture specific reasons, but best not leave the door open at
> all. ]

Can we give the -stable maintainers (and others) more explanation of
why they might choose to merge this?

> ...
>
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -1241,13 +1241,15 @@ void kthread_use_mm(struct mm_struct *mm)
>  	WARN_ON_ONCE(tsk->mm);
>  
>  	task_lock(tsk);
> +	local_irq_disable();

A bare local_irq_disable() is one of those "what the heck is this
protecting" things.  It's the new lock_kernel().

So a little comment will help readers to understand why we did it. 
Something like this?

--- a/kernel/kthread.c~mm-fix-kthread_use_mm-vs-tlb-invalidate-fix
+++ a/kernel/kthread.c
@@ -1239,6 +1239,7 @@ void kthread_use_mm(struct mm_struct *mm
 	WARN_ON_ONCE(tsk->mm);
 
 	task_lock(tsk);
+	/* Hold off tlb flush IPIs while switching mm's */
 	local_irq_disable();
 	active_mm = tsk->active_mm;
 	if (active_mm != mm) {
_

>  	active_mm = tsk->active_mm;
>  	if (active_mm != mm) {
>  		mmgrab(mm);
>  		tsk->active_mm = mm;
>  	}
>  	tsk->mm = mm;
> -	switch_mm(active_mm, mm, tsk);
> +	switch_mm_irqs_off(active_mm, mm, tsk);
> +	local_irq_enable();
>  	task_unlock(tsk);
>  #ifdef finish_arch_post_lock_switch
>  	finish_arch_post_lock_switch();
>
> ...
>