> So *some* saving can be achieved without moving stuff out.
I thought about that but fowner doesn't need to be in there at all.
And then we don't have to care about growing or shrinking the struct, or
its padding.
> > +struct fown_struct nop_fown_struct = {
> > + .lock = __RW_LOCK_UNLOCKED(nop_fown_struct.lock),
> > + .pid = NULL,
> > + .pid_type = PIDTYPE_MAX,
> > + .uid = INVALID_UID,
> > + .euid = INVALID_UID,
> > + .signum = 0,
> > +};
>
> why this instead of NULL checking?
Dedicated nop types makes it harder to cause accidental NULL
dereferences and currently checking whether a signal needs to be sent is
predicated on ->pid within fowner not being NULL. Which made a nop type
at least a viable option. I don't have a strong opinion.
> > +
> > +/*
> > + * Allocate an file->f_owner struct if it doesn't exist, handling racing
> > + * allocations correctly.
> > + */
> > +struct fown_struct *file_f_owner_allocate(struct file *file)
> > +{
> > + struct fown_struct *f_owner;
> > +
> > + f_owner = smp_load_acquire(&file->f_owner);
>
> For all spots of the sort you don't need an acquire fence, a consume
> fence is sufficient which I believe in Linux is guaranteed to be
> provided with READ_ONCE. I failed to find smp_load_consume, presumably
> for that reason.
Thanks.
> > +struct fown_struct *file_f_owner_allocate(struct file *file)
> > +{
> > + struct fown_struct *f_owner;
> > +
> > + f_owner = smp_load_acquire(&file->f_owner);
> > + if (f_owner != &nop_fown_struct)
> > + return NULL;
> > +
> > + f_owner = kzalloc(sizeof(struct fown_struct), GFP_KERNEL);
> > + if (!f_owner)
> > + return ERR_PTR(-ENOMEM);
> > +
> > + rwlock_init(&f_owner->lock);
> > + f_owner->file = file;
> > + /* If someone else raced us, drop our allocation. */
> > + if (unlikely(cmpxchg(&file->f_owner, &nop_fown_struct, f_owner) !=
> > + &nop_fown_struct)) {
> > + kfree(f_owner);
> > + return NULL;
>
> this wants to return the found pointer, not NULL
Caller was supposed to only see an allocation if they own it.
