linux-kernel - Re: [PATCH 3/5 v2] tracing: Automatically mount tracefs on debugfs/tracing

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Message-ID: <20150125195932.GH29656@ZenIV.linux.org.uk>
Date:	Sun, 25 Jan 2015 19:59:32 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/5 v2] tracing: Automatically mount tracefs on
 debugfs/tracing

On Sun, Jan 25, 2015 at 02:38:30PM -0500, Steven Rostedt wrote:
> On Sun, 25 Jan 2015 21:22:07 +0800
> Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
> 
> 
> > If we could do it in a non-racy way, that would be good, otherwise I
> > don't see us being able to even take this patch :(
> 
> Is it still racy even if it's only done at boot up? This path only gets
> hit the first time it is called. "if (tr->flags &TRACE_ARRAY_FL_GLOBAL)"
> is the top level tracing directory ("tracing") and is only called
> during boot up (fs_initcall) and never hit again. I could even make
> this called directly by that code so we could label it "__init" to make
> sure that it is to be never hit. Or is this racy even when done by
> fs_initcall?
> 
> Waiting for Al to comment on this, because, I can't add this feature
> until debugfs/tracing still containing the tracing information,
> otherwise it will break all the tools that interact with the tracing
> infrastructure, and we all know how happy Linus feels about such
> changes.

Actually, I'm almost done massaging that sucker into adding
debugfs_create_automount().  The only remaining question is what arguments
do we put it; for now I'm giving it dentry_operations + data (to go into
inode->i_private), but it might be better to give it a pointer just to
d_automount() callback + data for it...
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