[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <87wso0wa4c.fsf@saeurebad.de>
Date: Tue, 18 Mar 2008 12:21:39 +0100
From: Johannes Weiner <hannes@...urebad.de>
To: Jan Engelhardt <jengelh@...putergmbh.de>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: vfree with spin_lock_bh
Hi Jan,
Jan Engelhardt <jengelh@...putergmbh.de> writes:
> while transforming some code with big allocations (like 120 KB) from
> kmalloc to vmalloc — virtual contiguity is sufficient — I hit a
> BUG_ON in mm/vmalloc.c a number of times:
>
> void vfree(const void *addr)
> {
> BUG_ON(in_interrupt());
> __vunmap(addr, 1);
> }
>
> First I was thinking “how could iptables -F run in interrupt context?”,
> but apparently, it does seem to make a difference:
>
> ...
> spin_lock_bh(&a_local_spinlock);
> list_del_rcu(&node->list);
> printk(KERN_INFO "Interrupt? %lu\n", in_interrupt());
> /* vfree not worky here */
> spin_unlock_bh(&a_local_spinlock);
> printk(KERN_INFO "Interrupt? %lu\n", in_interrupt());
> /* now possible */
> vfree(node);
> ...
>
> and this gives (x86_32)
>
> Interrupt? 256
> Interrupt? 0
>
> So this may be a "property" of spinlocks, but it is a bit strange to me.
> Why should not I be able to call vfree() when I am, in fact, in
> user context (but with a bh spinlock held...).
in_interrupt() checks for both, hard- and softirqs. Since
spin_lock_bh() disables softirq's you have to be as fast as possible to
avoid softirq latency.
> Do I perhaps need a non-bh spinlock? There's RCU going on on that
> linked list so I am not sure whether I could just call the normal
> spin_lock() function.
Perhaps a call_rcu() which vfree()s the node? But I am just guessing
wildly here.
> Looking at the code of _spin_lock_bh in kernel/spinlock.c reveals that
> it is actually disabling preempt instead of being in an interrupt.
> Making an uneducated guess, would
>
> BUG_ON(in_interrupt() != 0 && in_interrupt() != 256)
That's basically a lacky in_irq(). The 256 you see here is
1<<SOFTIRQ_SHIFT but softirq disabling can be nested and you can not
check for 256 directly.
> in vfree() be safe?
Can not judge that.
Hannes
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists