Re: fs_struct refcounting: spinlock vs atomic

[Enrico Weigelt]

On 15.02.2018 10:14, Richard Weinberger wrote:
> On Wed, Feb 14, 2018 at 10:13 PM, Enrico Weigelt <lkml@xxxxxxxxx> wrote:
> > Hi folks,
> >
> >
> > in fork.c, a spinlock is held for fs_struct refcounting, while other
> > places - eg. switch_task_namespaces uses atomic_dec_and_test() on
> > the nsproxy.
> >
> > What's the exact difference here ? Could the atomic counting also used
> > for fs_struct ?
>
> Well, the spinlock protects more than just the counter. So atomic won't do it.

Okay. Is that needed in that case ?

See unshare() syscall:

if (new_fs) {
	fs = current->fs;
	spin_lock(&fs->lock);
	current->fs = new_fs;
	if (--fs->users)
		new_fs = NULL;
	else
		new_fs = fs;
	spin_unlock(&fs->lock);
}

Seems to me, that we're just refcounting here, and once it went dont to
zero, nobody else can access it anymore.


--mtx

--
Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
info@xxxxxxxxx -- +49-151-27565287