linux-kernel - Re: Preemptible idr

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Date:   Wed, 27 Jan 2021 00:41:24 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Courtney Cavin <courtney.cavin@...ymobile.com>
Subject: Re: Preemptible idr_alloc() in QRTR code

On Tue, Jan 26, 2021 at 06:36:02PM +0000, Mark Rutland wrote:
> On Tue, Jan 26, 2021 at 11:00:05AM -0600, Bjorn Andersson wrote:
> > On Tue 26 Jan 10:21 CST 2021, Mark Rutland wrote:
> > 
> > > On Tue, Jan 26, 2021 at 02:58:33PM +0000, Matthew Wilcox wrote:
> > > > On Tue, Jan 26, 2021 at 10:47:34AM +0000, Mark Rutland wrote:
> > > > > Hi,
> > > > > 
> > > > > When fuzzing arm64 with Syzkaller, I'm seeing some splats where
> > > > > this_cpu_ptr() is used in the bowels of idr_alloc(), by way of
> > > > > radix_tree_node_alloc(), in a preemptible context:
> > > > 
> > > > I sent a patch to fix this last June.  The maintainer seems to be
> > > > under the impression that I care an awful lot more about their
> > > > code than I do.
> > > > 
> > > > https://lore.kernel.org/netdev/20200605120037.17427-1-willy@infradead.org/
> > > 
> > > Ah; I hadn't spotted the (glaringly obvious) GFP_ATOMIC abuse, thanks
> > > for the pointer, and sorry for the noise.
> > > 
> > 
> > I'm afraid this isn't as obvious to me as it is to you. Are you saying
> > that one must not use GFP_ATOMIC in non-atomic contexts?
> > 
> > That said, glancing at the code I'm puzzled to why it would use
> > GFP_ATOMIC.
> 
> I'm also not entirely sure about the legitimacy of GFP_ATOMIC outside of
> atomic contexts -- I couldn't spot any documentation saying that wasn't
> legitimate, but Matthew's commit message implies so, and it sticks out
> as odd.

It's actually an assumption in the radix tree code.  If you say you
can't be preempted by saying GFP_ATOMIC, it takes you at your word and
does some things which cannot be done in preemptable context.