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Date: Wed, 23 Jan 2013 15:51:47 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [tracepoint] cargo-culting considered harmful...
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On Wed, 23 Jan 2013 22:55:24 +0000
Al Viro <viro@ZenIV.linux.org.uk> wrote:

> In samples/tracepoints/tracepoint-probe-sample.c:
> /*
>  * Here the caller only guarantees locking for struct file and struct inode.
>  * Locking must therefore be done in the probe to use the dentry.
>  */
> static void probe_subsys_event(void *ignore,   
>                                struct inode *inode, struct file *file)
> {
>         path_get(&file->f_path);
>         dget(file->f_path.dentry);
>         printk(KERN_INFO "Event is encountered with filename %s\n",
>                 file->f_path.dentry->d_name.name);
>         dput(file->f_path.dentry);
>         path_put(&file->f_path);
> }
> 
> note that
> 	* file->f_path is already pinned down by open(), path_get() does not
> provide anything extra.
> 	* file->f_path.dentry is already pinned by open() *and* path_get()
> just above that dget().
> 	* ->d_name.name *IS* *NOT* *PROTECTED* by pinning dentry down,
> whether it's done once or thrice.

I guess the first two are obvious (or at least, expected).  But the
third isn't.

Where should a kernel developer go to learn these things? 
include/linux/dcache.h doesn't mention d_name locking rules, nor does
Documentation/filesystems/vfs.txt.