linux-kernel - Re: freeze when strace X

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Date:	Wed, 9 Jun 2010 22:47:23 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Matthew Garrett <mjg59@...f.ucam.org>
cc:	Ortwin Glück <odi@....ch>,
	linux-kernel@...r.kernel.org
Subject: Re: freeze when strace X

On Wed, 9 Jun 2010, Matthew Garrett wrote:

> On Wed, Jun 09, 2010 at 10:02:21PM +0200, Ortwin Glück wrote:
> > On 09.06.2010 21:52, Matthew Garrett wrote:
> > > Just to make sure, you're not trying to strace X from within X, are you?
> > 
> > Oh... now that you mention it... Is that a silly thing to do?
> > strace X from a different terminal works fine :-)
> > 
> > But freezing the system because of that? I mean an error message, log, anything
> > would be "nice".
> 
> strace needs to output text via X, which generates output which needs to 
> be output via X... and you'll tend to deadlock. If X is holding any 
> interesting kernel locks at that point then it wouldn't surprise me if 
> the machine ends up dead.

Nothing to do with kernel locks. strace hooks into the syscall and
intercepts the data, so X is stopped at that point. Now try to send
output to a program which you stopped yourself :)

Thanks,

	tglx