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Date: Mon, 26 Jan 2015 21:46:58 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 00/16 v3] tracing: Add new file system tracefs
Message-ID: <20150126214658.GQ29656@ZenIV.linux.org.uk>
References: <20150126150913.653681560@goodmis.org>
 <20150126193049.GN29656@ZenIV.linux.org.uk>
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On Mon, Jan 26, 2015 at 03:42:59PM -0500, Steven Rostedt wrote:
> On Mon, 26 Jan 2015 19:30:49 +0000
> Al Viro <viro@ZenIV.linux.org.uk> wrote:
> 
> > You are still fighting an inconvenient API, but now it's not debugfs one -
> > it's your copy thereof.  Why not give your instances/ an inode_operations
> > of its own?  One with ->mkdir() and ->rmdir(), leaving all other directories
> > as-is.  That way you don't need the secondary methods at all.  And sure,
> > debugfs_create_dir() grabs ->i_mutex on parent, making you drop that in
> > your ->mkdir() if you want to call it.  But now you are not talking to it -
> > just to your own code, where you are free to change the calling conventions,
> > making it caller's responsibility to get that ->i_mutex.  The same goes for
> > the rmdir side...
> 
> The vfs layer grabs the i_mutex, which needs to be dropped.

What for?  Just keep it through your instance_mkdir/instance_rmdir and be
done with that.  Sure, it means that you need variants of file/directory
creation/removal primitives that would assume that parent is already locked
by caller (and would leave unlocking to the caller as well).  So add them...