linux-kernel - Re: [PATCH RESEND] tracing: Make tracing work when debugfs is not compiled or initialized.

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Message-ID: <20151106080233.GA4174@cac>
Date:	Fri, 6 Nov 2015 16:02:33 +0800
From:	Jiaxing Wang <hello.wjx@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] tracing: Make tracing work when debugfs is not
 compiled or initialized.

On Thu, Nov 05, 2015 at 10:54:32AM -0500, Steven Rostedt wrote:
> On Thu, 5 Nov 2015 13:23:01 +0800
> Jiaxing Wang <hello.wjx@...il.com> wrote:
> 
> > > > -	/*
> > > > -	 * As there may still be users that expect the tracing
> > > > -	 * files to exist in debugfs/tracing, we must automount
> > > > -	 * the tracefs file system there, so older tools still
> > > > -	 * work with the newer kerenl.
> > > > -	 */
> > > > -	tr->dir = debugfs_create_automount("tracing", NULL,
> > > > -					   trace_automount, NULL);
> > > > -	if (!tr->dir) {
> > > > -		pr_warn_once("Could not create debugfs directory 'tracing'\n");
> > > > -		return ERR_PTR(-ENOMEM);
> > > > +	if (debugfs_initialized()) {
> > > > +		/*
> > > > +		 * As there may still be users that expect the tracing
> > > > +		 * files to exist in debugfs/tracing, we must automount
> > > > +		 * the tracefs file system there, so older tools still
> > > > +		 * work with the newer kerenl.
> > > > +		 */
> > > > +		traced = debugfs_create_automount("tracing", NULL,
> > > > +						   trace_automount, NULL);
> > > > +		if (!traced)
> > > > +			pr_warn_once("Could not create debugfs directory 'tracing'\n");
> > > 
> > > This should return a warning, and please keep the tr->dir instead of
> > > this new traced variable.
> > Do you mean return ERR_PTR(-ENOMEM); when debugfs_create_automount()
> > return NULL?
> 
> Right.
> 
> > As long as tracefs is initialized, we can make tracing_init_dentry() return
> > NULL even if the debugfs automount point is not created(), and tracefs can
> > still be populated. If tracing_init_dentry() returns error in this case,
> > the caller of tracing_init_dentry() will not populate tracefs.
> 
> But this is still a failure. tracing_init_dentry() now only mounts
> tracefs on the debugfs/tracing directory. If it fails to do that when
> debugfs is available, then it should fail, as it would break backward
> compatibility with other tools.
> 
> If debugfs is not configured in, then it should set tr->dir to
> whatever (ENOMEM is fine), and return NULL.
> 
> 
> > > 
> > > >  	}
> > > >  
> > > > +	tr->dir = TRACE_TOP_DIR_ENTRY;
> > > > +
> > > 
> > > Also, no need to add this, because if debugfs is not initialize, then
> > > tr->dir would be ERR_PTR(-ENODEV), which still works as tr->dir is not
> > > NULL.
> > If we accept debugfs_create_automount() return NULL, tr->dir can still
> > be NULL if we do tr->dir = debugfs_create_automount().
> 
> What's wrong with that? This function is only to automount debugfs now.
> 
> Also, I think the first check should be:
> 
> 	if (WARN_ON(!tracefs_initialized()) ||
> 	    (IS_ENABLED(CONFIG_DEBUGFS) &&
> 	     WARN_ON(!debugfs_initialized()))
> 		return ERR_PTR(-ENODEV);
> 
> Then we don't need the if (debugfs_initialized()) conditional.

I will send you a new patch according to your suggestion, and if that
is OK, I will send a seperate patch to Greg KH to add stub for
debugfs_create_automount().

Thanks.
> 
> -- Steve
> 
> 
> > > 
> > > -- Steve
> > > 
> > > >  	return NULL;
> > > >  }
> > > >  
> > > 
> 
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