linux-kernel - Re: [RFC PATCH 0/4] Add hazard pointers to kernel

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Message-ID: <68e712df-49f7-474f-8761-6f34e992babd@amd.com>
Date: Fri, 20 Sep 2024 10:30:38 +0530
From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 John Johansen <john.johansen@...onical.com>
Cc: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
 rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...r.kernel.org,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Frederic Weisbecker <frederic@...nel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
 Joel Fernandes <joel@...lfernandes.org>,
 Josh Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
 Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Kent Overstreet <kent.overstreet@...il.com>, Vlastimil Babka
 <vbabka@...e.cz>, maged.michael@...il.com
Subject: Re: [RFC PATCH 0/4] Add hazard pointers to kernel

On 9/19/2024 12:16 PM, Linus Torvalds wrote:
> On Thu, 19 Sept 2024 at 00:44, Neeraj Upadhyay <Neeraj.Upadhyay@....com> wrote:
>>
>> While we were working on this problem, this refcount scalability issue got
>> resolved  recently with conditional ref acquisition [3] (however, there are new
>> developments in apparmor code which might bring back the refcount problem [4]).
> 
> Honestly, the various security layers should be a whole lot more
> careful about their horrid performance issues, and I think that [4]
> you point at needs to just be headed off at the pass.
> 
> No  more "the security layer is so bad at performance that we have to
> introduce new ref mechanisms", please. Let's push back on bad security
> layer code instead.
> 

Ok got it. Thanks for your feedback! I had tried using percpu refcount first
(in place of kref) in AppArmor. However, that required managing the last
reference drop (implemented in [1] and [2]). Mateusz has shared some ideas
in his reply to this thread. Maybe that is a workable solution. Will defer
to John on this as I have limited understanding of the cred management code.

- Neeraj

>                 Linus