linux-kernel - Re: lots of suspicious RCU traces

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Message-ID: <20121025055056.GA2661@swordfish>
Date:	Wed, 24 Oct 2012 22:50:56 -0700
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Frederic Weisbecker <fweisbec@...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

On (10/25/12 00:32), Frederic Weisbecker wrote:
> 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?
>

strace -f <anything>

 
	-ss
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