linux-kernel - Re: [PATCH v4 20/23] [RFC] kprobes: Remove task scan for updating kretprobe

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Message-ID: <20200828151806.GF1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 28 Aug 2020 17:18:06 +0200
From:   peterz@...radead.org
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     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
Subject: Re: [PATCH v4 20/23] [RFC] kprobes: Remove task scan for updating
 kretprobe_instance

On Sat, Aug 29, 2020 at 12:10:10AM +0900, Masami Hiramatsu wrote:
> On Fri, 28 Aug 2020 14:52:36 +0200
> peterz@...radead.org wrote:

> > >  	synchronize_rcu();
> > 
> > This one might help, this means we can do rcu_read_lock() around
> > get_kretprobe() and it's usage. Can we call rp->handler() under RCU?
> 
> Yes, as I said above, the get_kretprobe() (and kretprobe handler) must be
> called under preempt-disabled.

Then we don't need the ordering; we'll need READ_ONCE() (or
rcu_derefernce()) to make sure the address dependency works on Alpha.
And a comment/assertion that explains this might not go amiss in
get_kretprobe().