linux-kernel - Re: trace_printk is doing weird things with my arguments

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Date:	Mon, 12 Dec 2011 15:32:14 -0500
From:	Josef Bacik <josef@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Josef Bacik <josef@...hat.com>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: trace_printk is doing weird things with my arguments

On Mon, Dec 12, 2011 at 02:57:18PM -0500, Steven Rostedt wrote:
> On Mon, 2011-12-12 at 13:59 -0500, Josef Bacik wrote:
> > Hello,
> > 
> > I've been using trace_printk() to hash out where I want to put trace points so I
> > can do leak detection in the btrfs space accounting code.  This has worked out
> > wonderfully, except in the case where I have trace_printk()'s towards the end of
> > umount() which is where everything gets freed up.  After a lot of screwing
> > around I ended up with this in one of the functions
> > 
> > trace_printk("%pU is being released, fsinfo=%p\n", fs_info->fsid, fs_info);
> > printk(KERN_ERR "%pU is releaseing global_rsv\n", fs_info->fsid);
> > 
> > and then in my trace output I got this (I've cut the unnecessary line beginning)
> > 
> > 0080880d-0488-ffff-0a0a-0a0a0a0a0a0a is being released, fsinfo=ffff88040d88b000
> > 
> > and in dmesg I got this
> > 
> > 4e78b2a8-707a-4eef-97d5-11c0aa1b8f29 is releaseing global_rsv
> > 
> > The dmesg has the right fsid, and I have a ton of trace_printk()'s in
> > close_ctree() which is the function that we call from ->put_super, and all of
> > them will either randomly have the right fsid, they will all have the wrong
> > fsid, or some mixture, like half will have the right one but the last half will
> > not.  The '0a' repeating thing is because i put a memset(fs_info, 0xa,
> > sizeof(*fsinfo)) in our freeing function because I thought we were screwing up,
> > but it looks like trace_printk is somehow not trying to fill in the arguments
> > until after the arguments have been freed, which seems wrong and is screwing me
> > up ;).  Any thoughts?  Thanks,
> 
> Yes, that's the way trace_printk defaults to work. It really is a
> trace_bprintk(). In order for trace_printk to be as little impact as
> possible on recording, it only records as little as possible.
> 
> Although, I thought it would at least save the arguments at the point of
> time that they are recorded, this looks like it doesn't even do that.
> 
> Can you try this:
> 
> In your code after all the includes add:
> 
> #undef trace_printk
> #define trace_printk(fmt, args...) __trace_printk(_THIS_IP_, fmt, ##args)
> 
> This will process the format and arguments and place the final string
> right into the buffer. Normal printk will post process things if it can.
> Please let me know if the above works.
> 

That worked perfectly.  So will I have this problem when I got to turn these
things into tracepoints or will those work out right?  Thanks,

Josef
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