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Date: Wed, 24 Oct 2012 22:50:56 -0700
From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>, Dave Jones <davej@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Serge Hallyn <serge.hallyn@canonical.com>,
        linux-kernel@vger.kernel.org
Subject: Re: lots of suspicious RCU traces
Message-ID: <20121025055056.GA2661@swordfish>
References: <20121017034918.GA13295@redhat.com>
 <20121024164235.GA2467@swordfish>
 <20121024180608.GA22840@redhat.com>
 <20121024182111.GA2340@swordfish>
 <CAFTL4hxmh7yV8ePUhY_7hOuG=JrbZZcNvV88bWcUmwG9X0W04A@mail.gmail.com>
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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@gmail.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>

 
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