Re: [tracepoint] cargo-culting considered harmful...

Andrew Morton
2013-01-23T23:52:08Z

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.