linux-kernel - Re: [PATCH v2] mm: Fix use

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Date:   Wed, 26 Feb 2020 07:13:37 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, luto@...capital.net,
        keescook@...omium.org, torvalds@...ux-foundation.org,
        jannh@...gle.com, will@...nel.org
Subject: Re: [PATCH v2] mm: Fix use_mm() vs TLB invalidate

On 2/26/20 6:21 AM, Peter Zijlstra 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. ]

Not that I'm worried about it breaking anything, but ran it through
the usual testing and might as well report:

Tested-by: Jens Axboe <axboe@...nel.dk>

-- 
Jens Axboe