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Date:   Thu, 7 Jul 2022 17:57:50 +0100
From:   Aaron Tomlin <atomlin@redhat.com>
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     rostedt@goodmis.org, cl@linux.com, pmladek@suse.com,
        mbenes@suse.cz, christophe.leroy@csgroup.eu,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-modules@vger.kernel.org, atomlin@atomlin.com,
        ghalat@redhat.com, oleksandr@natalenko.name, neelx@redhat.com,
        daniel.thompson@linaro.org, hch@infradead.org, tglx@linutronix.de,
        linux-rt-users@vger.kernel.org
Subject: Re: [PATCH v2] module: kallsyms: Ensure preemption in add_kallsyms()
 with PREEMPT_RT
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References: <20220704161753.4033684-1-atomlin@redhat.com>
 <YsXNVSAtO+VDggcI@bombadil.infradead.org>
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On Wed 2022-07-06 10:58 -0700, Luis Chamberlain wrote:
> Hey Aaron, thanks again!

Hi Luis,

No problem :)

> On Mon, Jul 04, 2022 at 05:17:53PM +0100, Aaron Tomlin wrote:
> > To disable preemption in the context of add_kallsyms() is incorrect.
> 
> Why, what broke? Did this used to work? Was the commit in question a
> regression then? Clarifying all this will help a lot.

Sorry for the confusion! If I understand correctly, nothing broke
intrinsically.

Rather with commit 08126db5ff73 ("module: kallsyms: Fix suspicious rcu
usage") under PREEMPT_RT=y, by disabling preemption, I introduced an
unbounded latency since the loop is not fixed which is generally frowned
upon. So, I would say this was a regression since earlier preemption was
not disabled and we would dereference RCU-protected pointers explicitly
i.e. without using the more appropriate rcu_dereference() family
of primitives. That being said, these pointers cannot change in this
context as explained previously.

Would the above be suitable - just to confirm before I send another
iteration?


Kind regards,

-- 
Aaron Tomlin