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Date: Fri, 19 Oct 2018 08:30:54 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] userfaultfd: disable irqs when taking the waitqueue
 lock
Message-ID: <20181019063054.GA28667@lst.de>
References: <20181018154101.18750-1-hch@lst.de> <20181018155611.42b9977ba08419b7869619c5@linux-foundation.org>
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>, linux-mm@kvack.org

On Thu, Oct 18, 2018 at 03:56:11PM -0700, Andrew Morton wrote:
> On Thu, 18 Oct 2018 17:41:01 +0200 Christoph Hellwig <hch@lst.de> wrote:
> 
> > userfaultfd contains howe-grown locking of the waitqueue lock,
> > and does not disable interrupts.  This relies on the fact that
> > no one else takes it from interrupt context and violates an
> > invariat of the normal waitqueue locking scheme.  With aio poll
> > it is easy to trigger other locks that disable interrupts (or
> > are called from interrupt context).
> 
> So...  this is needed in 4.19.x but not earlier?  Or something else?

Yes.