linux-kernel - Re: [RFC][PATCH] making vfree() safe from interrupt contexts

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Date:	Sun, 3 Mar 2013 15:01:43 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [RFC][PATCH] making vfree() safe from interrupt contexts

On Sun, Mar 3, 2013 at 2:51 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> +struct vfree_deferred {
> +       spinlock_t lock;
> +       void *list;
> +       struct work_struct wq;
> +};

Looking more at this, just get rid of the spinlock entirely, and use
<linux/llist.h> for the list.

IRQ-safety without the locking. Because you got the locking wrong
again, and made free_work() use spin_lock_irq(), but

> +static inline void deferred_vfree(void *addr)
> +{
> +       struct vfree_deferred *p = &get_cpu_var(vfree_deferred);
> +       spin_lock(&p->lock);

This needs to be a spin_lock_irqsave() too.

> +       *(void **)addr = p->list;
> +       p->list = addr;
> +       schedule_work(&p->wq);
> +       spin_unlock(&p->lock);
> +       put_cpu_var(vfree_deferred);

And there is no reason to hold the lock - or even stay on the CPU -
over the work-scheduling (which had better be irq- and smp-safe on its
own anyway), so you're actually best off just using

    struct vfree_deferred *p = &__get_cpu_var(vfree_deferred);
    struct llist_node *new = (void *)addr;

    llist_add(new, &p->list);
    schedule_work(&p->wq);

and you're done.

I'm not even sure it's worth it making it per-cpu, but I guess it
won't hurt either.

                  Linus
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