Date: Mon, 14 Nov 2016 12:30:40 -0800
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexei Starovoitov <ast@fb.com>, Dave Jones <davej@codemonkey.org.uk>,
        Martin KaFai Lau <kafai@fb.com>, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] tracing: ignore FTRACE_FL_DISABLED while walking
 dyn_ftrace records
Message-ID: <20161114203039.GA1646@ast-mbp.thefacebook.com>
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 <20161114142428.1091961b@gandalf.local.home>
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On Mon, Nov 14, 2016 at 02:24:28PM -0500, Steven Rostedt wrote:
> On Mon, 7 Nov 2016 15:14:20 -0800
> Alexei Starovoitov <ast@fb.com> wrote:
> 
> > ftrace_shutdown() checks for sanity of ftrace records
> > and if dyn_ftrace->flags is not zero, it will warn.
> > It can happen that 'flags' are set to FTRACE_FL_DISABLED at this point,
> > since some module was loaded, but before ftrace_module_enable()
> > cleared the flags for this module.
> > 
> > In other words the module.c is doing:
> > ftrace_module_init(mod); // calls ftrace_update_code() that sets flags=FTRACE_FL_DISABLED
> > ... // here ftrace_shutdown() is called that warns, since
> > err = prepare_coming_module(mod); // didn't have a chance to clear FTRACE_FL_DISABLED
> > 
> > Fix it by ignoring disabled records.
> > It's similar to what __ftrace_hash_rec_update() is already doing.
> > 
> 
> Alexei,
> 
> Do you have a clear reproducer of this bug?

Nope. No reproducer. It's very rare. Only stack trace that I posted earlier.
Since it kills ftrace everything that depends on it later spews a ton
of other warnings. So it's an important fix.