linux-kernel - Re: [PATCH v5 00/21] kprobes: Unify kretprobe trampoline handlers and make kretprobe lockless

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Message-Id: <20200909142840.b2245ae2f8325f042a3bc546@kernel.org>
Date:   Wed, 9 Sep 2020 14:28:40 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     peterz@...radead.org, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org, Eddy_Wu@...ndmicro.com,
        x86@...nel.org, davem@...emloft.net, rostedt@...dmis.org,
        naveen.n.rao@...ux.ibm.com, anil.s.keshavamurthy@...el.com,
        linux-arch@...r.kernel.org, cameron@...dycamel.com,
        oleg@...hat.com, will@...nel.org, paulmck@...nel.org,
        systemtap@...rceware.org
Subject: Re: [PATCH v5 00/21] kprobes: Unify kretprobe trampoline handlers
 and make kretprobe lockless

On Wed, 9 Sep 2020 00:09:23 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> > > Of course make it lockless then warning is gone.
> > > But even without the lockless patch, this warning can be false-positive
> > > because we prohibit nested kprobe call, right?
> > 
> > Yes, because the actual nesting is avoided by kprobe_busy, but lockdep
> > can't tell. Lockdep sees a regular lock user and an in-nmi lock user and
> > figures that's a bad combination.

Hmm, what about introducing new LOCK_USED_KPROBE bit, which will be set
if the lock is accessed when the current_kprobe is set (including kprobe_busy)?
This means it is in the kprobe user-handler context. If we access the lock always
in the kprobes context, it is never nested.

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>