linux-kernel - Re: lots of suspicious RCU traces

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Message-ID: <CAFTL4hxmh7yV8ePUhY_7hOuG=JrbZZcNvV88bWcUmwG9X0W04A@mail.gmail.com>
Date:	Thu, 25 Oct 2012 00:32:14 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Dave Jones <davej@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-kernel@...r.kernel.org
Subject: Re: lots of suspicious RCU traces

First of all, thanks a lot for your report.

2012/10/24 Sergey Senozhatsky <sergey.senozhatsky@...il.com>:
> On (10/24/12 20:06), Oleg Nesterov wrote:
>> On 10/24, Sergey Senozhatsky wrote:
>> >
>> > small question,
>> >
>> > ptrace_notify() and forward calls are able to both indirectly and directly call schedule(),
>> > /* direct call from ptrace_stop()*/,
>> > should, in this case, rcu_user_enter() be called before tracehook_report_syscall_exit(regs, step)
>> > and ptrace chain?
>>
>> Well, I don't really understand this magic... but why?
>>
>
> My understanding is (I may be wrong) that we can schedule() from ptrace chain to
> some arbitrary task, which will continue its execution from the point where RCU assumes
> CPU as not idle, while CPU in fact still in idle state -- no one said rcu_idle_exit()
> (or similar) prior to schedule() call.

Yeah but when we are in syscall_trace_leave(), the CPU shouldn't be in
RCU idle mode. That's where the bug is. How do you manage to trigger
this bug?

>
> if so, does the same apply to in_user?
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