linux-kernel - Re: __ftrace_hash_rec_update FTRACE_WARN

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Message-ID: <1374778741.3356.181.camel@gandalf.local.home>
Date:	Thu, 25 Jul 2013 14:59:01 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Steve Hodgson <steve@...estorage.com>
Cc:	Jörn Engel <joern@...fs.org>,
	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: __ftrace_hash_rec_update FTRACE_WARN_ON.

On Wed, 2013-07-24 at 22:46 -0700, Steve Hodgson wrote:

> No, nothing so complicated.
> 
> I added symbols from a kernel module into
> /sys/kernel/debug/tracing/set_ftrace_filter, enabled tracing, then
> unloaded the kernel module.
> 
> Which I'll admit is a stupid thing to do. I was then punished for my
> stupidity by being unable to use ftrace until the next reboot.

It's not stupid. ftrace should be robust enough to handle this. Anyway,
thank you, I found a reproducer.

Removing the module isn't good enough. You need to load it again.

 # cd /sys/kernel/debug/tracing
 # cat set_ftrace_filter
#### all functions enabled ####
 # modprobe uinput
 # echo uniput_release > set_ftrace_filter
 # cat set_ftrace_filter
uinput_release [uinput]
 # rmmod uinput
 # cat set_ftrace_filter
 # # note this is empty
 # modprobe uinput
 # cat set_ftrace_filter
uinput_release [uinput]
 # # '<it magically appeared>'
 # echo > set_ftrace_filter
[BOOM]

Now that I know what's the problem, it shouldn't be too hard to fix.

Thanks!

-- Steve


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