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

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Date:   Fri, 21 Aug 2020 15:04:45 +0200
From:   peterz@...radead.org
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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 Fri, Aug 21, 2020 at 11:09:51AM +0530, Aneesh Kumar K.V wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> 
> > 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. ]
> 
> 
> Do we have similar race with exec_mmap()? I am looking at exec_mmap()
> runnning parallel to do_exit_flush_lazy_tlb(). We can get
> 
> 	if (current->active_mm == mm) {
> 
> true and if we don't disable irq around updating tsk->mm/active_mm we
> can end up doing mmdrop on wrong mm?

exec_mmap() is called after de_thread(), there should not be any mm
specific invalidations around I think.

Then again, CLONE_VM without CLONE_THREAD might still be possible, so
yeah, we probably want IRQs disabled there too, just for consistency and
general paranoia if nothing else.