linux-kernel - [tracepoint] cargo-culting considered harmful...

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Message-ID: <20130123225523.GY4939@ZenIV.linux.org.uk>
Date:	Wed, 23 Jan 2013 22:55:24 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [tracepoint] cargo-culting considered harmful...

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 do realize that it's just an example, but perhaps we should rename that
file to match the contents?  The only question is whether it should be
git mv samples/tracepoints/{tracepoint-probe-sample,cargo-cult}.c
or git mv samples cargo-cult...

					Al, seriously peeved.
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