linux-kernel - Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value

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Date:   Fri, 30 Aug 2019 08:43:22 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vladimir Davydov <vdavydov@...allels.com>,
        Kirill Tkhai <ktkhai@...allels.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference()
 without checking return value

On Fri, Aug 30, 2019 at 8:40 AM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> Ah, ok.  Might be worth some comments - I find the comments in that
> function particularly unhelpful (even after Oleg mentions this is
> case 2.)

Yeah, a comment like "This might fault if we race with the task
scheduling away and being destroyed, but we check that below".

But that code has some performance issues too, as mentioned. Which is
all kinds of sad since it clearly _tries_ to perform well with RCU
locking and optimistic accesses etc.

             Linus