linux-kernel - Re: lots of suspicious RCU traces

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Message-ID: <20121024185230.GB5025@redhat.com>
Date:	Wed, 24 Oct 2012 20:52:30 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Dave Jones <davej@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.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 10/24, Sergey Senozhatsky wrote:
>
> 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)

Oh, I bet I have much more chances to be wrong ;)

> that we can schedule() from ptrace chain to

I don't understand how ptrace chain differs from, say, audit_syscall_exit().
There is nothing special in ptrace_stop() in this respect.

> 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()

confused... of course it would be wrong if syscall_trace_leave() is
called when CPU is considered idle,

> if so, does the same apply to in_user?

Not sure we understand each other. But I believe that ->in_user should be
already false when syscall_trace_leave() is called.

Oleg.

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