On Thu, 1 Feb 2024 00:27:19 +0000
Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Jan 31, 2024 at 01:49:22PM -0500, Steven Rostedt wrote:
>
> > @@ -329,32 +320,29 @@ static struct dentry *create_file(const char *name, umode_t mode,
> >
> > ti = get_tracefs(inode);
> > ti->flags |= TRACEFS_EVENT_INODE;
> > - d_instantiate(dentry, inode);
> > +
> > + d_add(dentry, inode);
> > fsnotify_create(dentry->d_parent->d_inode, dentry);
>
> Seriously? stat(2), have it go away from dcache on memory pressure,
> lather, rinse, repeat... Won't *snotify get confused by the stream
> of creations of the same thing, with not a removal in sight?
>
That looks to be cut and paste from the old create in tracefs. I don't know
of a real use case for that. I think we could possibly delete it without
anyone noticing.
> > - return eventfs_end_creating(dentry);
> > + return dentry;
> > };
> >
> > @@ -371,11 +359,14 @@ static struct dentry *create_dir(struct eventfs_inode *ei, struct dentry *parent
> > /* Only directories have ti->private set to an ei, not files */
> > ti->private = ei;
> >
> > + dentry->d_fsdata = ei;
> > + ei->dentry = dentry; // Remove me!
> > +
> > inc_nlink(inode);
> > - d_instantiate(dentry, inode);
> > + d_add(dentry, inode);
> > inc_nlink(dentry->d_parent->d_inode);
>
> What will happen when that thing gets evicted from dcache,
> gets looked up again, and again, and...?
>
> > fsnotify_mkdir(dentry->d_parent->d_inode, dentry);
>
> Same re snotify confusion...
Yeah, again, I think it's useless. Doing that is more useless than taring
the tracefs directory ;-)
>
> > - return eventfs_end_creating(dentry);
> > + return dentry;
> > }
> >
> > static void free_ei(struct eventfs_inode *ei)
> > @@ -425,7 +416,7 @@ void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry)
> > }
> >
> > @@ -607,79 +462,55 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
> > struct dentry *dentry,
> > unsigned int flags)
> > {
> > - const struct file_operations *fops;
> > - const struct eventfs_entry *entry;
> > struct eventfs_inode *ei_child;
> > struct tracefs_inode *ti;
> > struct eventfs_inode *ei;
> > - struct dentry *ei_dentry = NULL;
> > - struct dentry *ret = NULL;
> > - struct dentry *d;
> > const char *name = dentry->d_name.name;
> > - umode_t mode;
> > - void *data;
> > - int idx;
> > - int i;
> > - int r;
> > + struct dentry *result = NULL;
> >
> > ti = get_tracefs(dir);
> > if (!(ti->flags & TRACEFS_EVENT_INODE))
>
> Can that ever happen? I mean, why set ->i_op to something that
> has this for ->lookup() on a directory without TRACEFS_EVENT_INODE in
> its inode? It's not as if you ever removed that flag...
That's been there mostly as paranoia. Should probably be switched to:
if (WARN_ON_ONCE(!(ti->flags & TRACEFS_EVENT_INODE)))
>
> > - return NULL;
> > -
> > - /* Grab srcu to prevent the ei from going away */
> > - idx = srcu_read_lock(&eventfs_srcu);
> > + return ERR_PTR(-EIO);
> >
> > - /*
> > - * Grab the eventfs_mutex to consistent value from ti->private.
> > - * This s
> > - */
> > mutex_lock(&eventfs_mutex);
> > - ei = READ_ONCE(ti->private);
> > - if (ei && !ei->is_freed)
> > - ei_dentry = READ_ONCE(ei->dentry);
> > - mutex_unlock(&eventfs_mutex);
> > -
> > - if (!ei || !ei_dentry)
> > - goto out;
> >
> > - data = ei->data;
> > + ei = ti->private;
> > + if (!ei || ei->is_freed)
> > + goto enoent;
> >
> > - list_for_each_entry_srcu(ei_child, &ei->children, list,
> > - srcu_read_lock_held(&eventfs_srcu)) {
> > + list_for_each_entry(ei_child, &ei->children, list) {
> > if (strcmp(ei_child->name, name) != 0)
> > continue;
> > - ret = simple_lookup(dir, dentry, flags);
> > - if (IS_ERR(ret))
> > - goto out;
> > - d = create_dir_dentry(ei, ei_child, ei_dentry);
> > - dput(d);
> > + if (ei_child->is_freed)
> > + goto enoent;
>
> Out of curiosity - can that happen now? You've got exclusion with
> eventfs_remove_rec(), so you shouldn't be able to catch the moment
> between setting ->is_freed and removal from the list...
Yeah, that's from when we just used SRCU. If anything, it too should just
add a WARN_ON_ONCE() to it.
>
> > + lookup_dir_entry(dentry, ei, ei_child);
> > goto out;
> > }
> >
> > - for (i = 0; i < ei->nr_entries; i++) {
> > - entry = &ei->entries[i];
> > - if (strcmp(name, entry->name) == 0) {
> > - void *cdata = data;
> > - mutex_lock(&eventfs_mutex);
> > - /* If ei->is_freed, then the event itself may be too */
> > - if (!ei->is_freed)
> > - r = entry->callback(name, &mode, &cdata, &fops);
> > - else
> > - r = -1;
> > - mutex_unlock(&eventfs_mutex);
> > - if (r <= 0)
> > - continue;
> > - ret = simple_lookup(dir, dentry, flags);
> > - if (IS_ERR(ret))
> > - goto out;
> > - d = create_file_dentry(ei, i, ei_dentry, name, mode, cdata, fops);
> > - dput(d);
> > - break;
> > - }
> > + for (int i = 0; i < ei->nr_entries; i++) {
> > + void *data;
> > + umode_t mode;
> > + const struct file_operations *fops;
> > + const struct eventfs_entry *entry = &ei->entries[i];
> > +
> > + if (strcmp(name, entry->name) != 0)
> > + continue;
> > +
> > + data = ei->data;
> > + if (entry->callback(name, &mode, &data, &fops) <= 0)
> > + goto enoent;
> > +
> > + lookup_file_dentry(dentry, ei, i, mode, data, fops);
> > + goto out;
> > }
> > +
> > + enoent:
> > + /* Don't cache negative lookups, just return an error */
> > + result = ERR_PTR(-ENOENT);
>
> Huh? Just return NULL and be done with that - you'll get an
> unhashed negative dentry and let the caller turn that into
> -ENOENT...
We had a problem here with just returning NULL. It leaves the negative
dentry around and doesn't get refreshed.
I did this:
# cd /sys/kernel/tracing
# ls events/kprobes/sched/
ls: cannot access 'events/kprobes/sched/': No such file or directory
# echo 'p:sched schedule' >> kprobe_events
# ls events/kprobes/sched/
ls: cannot access 'events/kprobes/sched/': No such file or directory
When it should have been:
# ls events/kprobes/sched/
enable filter format hist hist_debug id inject trigger
Leaving the negative dentry there will have it fail when the directory
exists the next time.
-- Steve
>
> > out:
> > - srcu_read_unlock(&eventfs_srcu, idx);
> > - return ret;
> > + mutex_unlock(&eventfs_mutex);
> > + return result;
> > }
> >
> > /*
